diff --git a/README.md b/README.md
index 5145386..57aec3c 100644
--- a/README.md
+++ b/README.md
@@ -229,8 +229,10 @@ public class Test {
         captcha.setLen(3);  // 几位数运算，默认是两位
         captcha.getArithmeticString();  // 获取运算的公式：3+2=?
         captcha.text();  // 获取运算的结果：5
-        
+        captcha.supportAlgorithmSign(2); // 可设置支持的算法：2 表示只生成带加减法的公式
+        captcha.setDifficulty(50); // 设置计算难度，参与计算的每一个整数的最大值
         captcha.out(outputStream);  // 输出验证码
+        //简单算术类型 SimpleArithmeticCaptcha,用法同ArithmeticCaptcha,只支持加减，计算结果为正整数
     }
 }
 ```
diff --git a/src/main/java/com/wf/captcha/SimpleArithmeticCaptcha.java b/src/main/java/com/wf/captcha/SimpleArithmeticCaptcha.java
new file mode 100644
index 0000000..c393f31
--- /dev/null
+++ b/src/main/java/com/wf/captcha/SimpleArithmeticCaptcha.java
@@ -0,0 +1,160 @@
+package com.wf.captcha;
+
+import com.wf.captcha.base.ArithmeticCaptchaAbstract;
+
+import javax.imageio.ImageIO;
+import javax.script.ScriptEngine;
+import javax.script.ScriptEngineManager;
+import javax.script.ScriptException;
+import java.awt.*;
+import java.awt.image.BufferedImage;
+import java.io.IOException;
+import java.io.OutputStream;
+
+/**
+ * Description：最后一位计算时保证结果为正，避免负号输入
+ * 2位3位整数计算比较好,否则最后一位会数字会有点异常
+ *
+ * @author Li Haoran
+ */
+public class SimpleArithmeticCaptcha extends ArithmeticCaptchaAbstract {
+
+    public SimpleArithmeticCaptcha() {
+    }
+
+    public SimpleArithmeticCaptcha(int width, int height) {
+        this();
+        setWidth(width);
+        setHeight(height);
+    }
+
+    public SimpleArithmeticCaptcha(int width, int height, int len) {
+        this(width, height);
+        setLen(len);
+    }
+
+    public SimpleArithmeticCaptcha(int width, int height, int len, Font font) {
+        this(width, height, len);
+        setFont(font);
+    }
+
+    public SimpleArithmeticCaptcha(int width, int height, int len, int  font) throws IOException, FontFormatException {
+        this(width, height, len);
+        setFont(font,32f);
+    }
+
+    /**
+     * 生成验证码
+     *
+     * @param out 输出流
+     * @return 是否成功
+     */
+    @Override
+    public boolean out(OutputStream out) {
+        checkAlpha();
+        return graphicsImage(getArithmeticString().toCharArray(), out);
+    }
+
+    @Override
+    public String toBase64() {
+        return toBase64("data:image/png;base64,");
+    }
+
+    /**
+     * 生成验证码图形
+     *
+     * @param strs 验证码
+     * @param out  输出流
+     * @return boolean
+     */
+    private boolean graphicsImage(char[] strs, OutputStream out) {
+        try {
+            BufferedImage bi = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
+            Graphics2D g2d = (Graphics2D) bi.getGraphics();
+            // 填充背景
+            g2d.setColor(Color.WHITE);
+            g2d.fillRect(0, 0, width, height);
+            // 抗锯齿
+            g2d.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
+            // 画干扰圆
+            drawOval(2, g2d);
+            // 画字符串
+            g2d.setFont(getFont());
+            FontMetrics fontMetrics = g2d.getFontMetrics();
+            int fW = width / strs.length;  // 每一个字符所占的宽度
+            int fSp = (fW - (int) fontMetrics.getStringBounds("8", g2d).getWidth()) / 2;  // 字符的左右边距
+            for (int i = 0; i < strs.length; i++) {
+                g2d.setColor(color());
+                int fY = height - ((height - (int) fontMetrics.getStringBounds(String.valueOf(strs[i]), g2d).getHeight()) >> 1);  // 文字的纵坐标
+                g2d.drawString(String.valueOf(strs[i]), i * fW + fSp + 3, fY - 3);
+            }
+            g2d.dispose();
+            ImageIO.write(bi, "png", out);
+            out.flush();
+            return true;
+        } catch (IOException e) {
+            e.printStackTrace();
+        } finally {
+            try {
+                out.close();
+            } catch (IOException e) {
+                e.printStackTrace();
+            }
+        }
+        return false;
+    }
+
+    /**
+     * 生成随机验证码
+     * 不包含乘法、没有负数
+     * @return 验证码字符数组
+     */
+    @Override
+    protected char[] alphas() {
+        StringBuilder sb = new StringBuilder();
+        for (int i = 0; i < len-1; i++) {
+            sb.append(num(difficulty));
+            if (i < len - 1) {
+                int type = num(1, 3);
+                if (type == 1) {
+                    sb.append("+");
+                } else if (type == 2) {
+                    sb.append("-");
+                }
+            }
+        }
+        ScriptEngineManager manager = new ScriptEngineManager();
+        ScriptEngine engine = manager.getEngineByName("javascript");
+        try {
+            chars = String.valueOf(engine.eval(sb.toString()));
+        } catch (ScriptException e) {
+            e.printStackTrace();
+        }
+
+        int parseInt = Integer.parseInt(chars);
+        if(parseInt<0){
+            sb.append("+");
+            sb.append(num(-parseInt,-parseInt+11));
+        }else {
+            int type = num(1, 3);
+            if (type == 1) {
+                sb.append("+");
+                sb.append(num(difficulty));
+            } else if (type == 2) {
+                sb.append("-");
+                sb.append(num(0,parseInt+1));
+            }
+        }
+
+        try {
+            chars = String.valueOf(engine.eval(sb.toString()));
+        } catch (ScriptException e) {
+            e.printStackTrace();
+        }
+
+        sb.append("=?");
+        setArithmeticString(sb.toString());
+        return chars.toCharArray();
+    }
+
+}
diff --git a/src/main/java/com/wf/captcha/base/ArithmeticCaptchaAbstract.java b/src/main/java/com/wf/captcha/base/ArithmeticCaptchaAbstract.java
index daea140..490d8e8 100644
--- a/src/main/java/com/wf/captcha/base/ArithmeticCaptchaAbstract.java
+++ b/src/main/java/com/wf/captcha/base/ArithmeticCaptchaAbstract.java
@@ -11,6 +11,9 @@ import javax.script.ScriptException;
 public abstract class ArithmeticCaptchaAbstract extends Captcha {
     private String arithmeticString;  // 计算公式
 
+    protected static int difficulty = 10;
+    protected static int algorithmSign = 4;
+
     public ArithmeticCaptchaAbstract() {
         setLen(2);
     }
@@ -24,9 +27,9 @@ public abstract class ArithmeticCaptchaAbstract extends Captcha {
     protected char[] alphas() {
         StringBuilder sb = new StringBuilder();
         for (int i = 0; i < len; i++) {
-            sb.append(num(10));
+            sb.append(num(difficulty));
             if (i < len - 1) {
-                int type = num(1, 4);
+                int type = num(1, algorithmSign);
                 if (type == 1) {
                     sb.append("+");
                 } else if (type == 2) {
@@ -56,4 +59,18 @@ public abstract class ArithmeticCaptchaAbstract extends Captcha {
     public void setArithmeticString(String arithmeticString) {
         this.arithmeticString = arithmeticString;
     }
+
+    public void setDifficulty(int difficulty) {
+        this.difficulty = difficulty;
+    }
+
+    /**
+     * algorithmSign 2 : 支持加法
+     * algorithmSign 3 : 支持加减法
+     * algorithmSign 4 : 支持加减乘法
+     * @param algorithmSign 计算公式标示
+     */
+    public void supportAlgorithmSign(int algorithmSign){
+        this.algorithmSign = algorithmSign;
+    }
 }
diff --git a/src/test/java/com/wf/captcha/CaptchaTest.java b/src/test/java/com/wf/captcha/CaptchaTest.java
index d4ee3d5..27df97a 100644
--- a/src/test/java/com/wf/captcha/CaptchaTest.java
+++ b/src/test/java/com/wf/captcha/CaptchaTest.java
@@ -56,6 +56,8 @@ public class CaptchaTest {
         /*for (int i = 0; i < 10; i++) {
             ArithmeticCaptcha specCaptcha = new ArithmeticCaptcha();
             specCaptcha.setLen(3);
+            specCaptcha.supportAlgorithmSign(2);
+            specCaptcha.setDifficulty(50);
             specCaptcha.setFont(i, 28f);
             System.out.println(specCaptcha.getArithmeticString() + " " + specCaptcha.text());
             specCaptcha.out(new FileOutputStream(new File("C:/Java/aa" + i + ".png")));