直接貼程式碼
js部分

function randomRgbColor() { //隨機生成RGB顏色
            var r = Math.floor(Math.random() * 256); //隨機生成256以內r值
            var g = Math.floor(Math.random() * 256); //隨機生成256以內g值
            var b = Math.floor(Math.random() * 256); //隨機生成256以內b值
            return "rgb(" + r + "," + g + "," + b + ")"
 
; //返回rgb(r,g,b)格式顏色
        }
        window.onload = function () {
            draw();
            document.getElementById("changeImg").onclick=function(){
                draw();
            }
        }

        function draw(){
            var canvas = document.getElementById("canvas");
            var
 
 context = canvas.getContext("2d");
            canvas.width = 120;
            canvas.height = 40;
            context.strokeRect(0, 0, 120, 40);
            var aCode = "A,B,C,E,F,G,H,J,K,L,M,N,P,Q,R,S,T,W,X,Y,1,2,3,4,5,6,7,8,9";
            console.log(aCode.split(","));
            var aLength = aCode.split(","
 
).length;
            for (var i = 0; i <= 3; i++) {
                var x = 20 + i * 20;
                var y = 20 + Math.random() * 10;
                var j = Math.floor(Math.random() * aLength);
                var deg = Math.random() * 90 * Math.PI / 180;//隨機弧度
                var txt = aCode.split(",")[j];
                context.fillStyle = randomRgbColor();
                context.font = "bold 20px 微軟雅黑";
                //修改座標原點和旋轉角度
                context.translate(x, y);
                context.rotate(deg);
                context.fillText(txt, 0, 0);
                //恢復座標原點和旋轉角度
                context.rotate(-deg);
                context.translate(-x, -y);
            }
            //干擾線
            for (var i = 0; i < 8; i++) {
                context.strokeStyle = randomRgbColor();
                context.beginPath();
                context.moveTo(Math.random() * 120, Math.random() * 40);
                context.lineTo(Math.random() * 120, Math.random() * 40);
                context.stroke();
            }
            /**繪製干擾點**/
            for (var i = 0; i < 20; i++) {
                context.fillStyle = randomRgbColor();
                context.beginPath();
                context.arc(Math.random() * 120, Math.random() * 40, 1, 0, 2 * Math.PI);
                context.fill();
            }
        }

html部分

 <canvas id="canvas"></canvas>
 <a href="javascript:;" id="changeImg">再換一張</a>

不知道為什麼不能上傳圖片了，請自行測試。