<script type="text/javascript">

        var searchLight={x:400,y:400,radius:150,vx:Math.random()*5+10,vy:Math.random()*5+10};

window.onload=function(){

var canvas=document.getElementById('canvas');

var context=canvas.getContext('2d');

            canvas.width=800;

            canvas.height=800;

            setInterval(function(){

            draw(context);

            update(canvas.width,canvas.height);

            },50);

}

function draw(cxt){

var canvas=cxt.canvas;

cxt.clearRect(0,0,canvas.width,canvas.height);

cxt.save();

            cxt.beginPath();

cxt.fillStyle="#000";

cxt.fillRect(0,0,canvas.width,canvas.height);

cxt.beginPath();

cxt.arc(searchLight.x,searchLight.y,searchLight.radius,0,Math.PI*2);

cxt.fillStyle="#FFF";

cxt.fill();

cxt.clip();

cxt.font="bold 150px Arial";

cxt.textAlign="center";

cxt.textBaseLine="middle";

cxt.fillStyle="#ff6600";

cxt.fillText("CANVAS",canvas.width/2,canvas.width/4);

cxt.fillText("CANVAS",canvas.width/2,canvas.width/2);

cxt.fillText("CANVAS",canvas.width/2,canvas.width*3/4);

cxt.restore();

}

function update(canvasWidth,canvasHeight){

   searchLight.x+=searchLight.vx;

   searchLight.y+=searchLight.vy;

  if(searchLight.x-searchLight.radius<=0){

              searchLight.vx=-searchLight.vx;

              searchLight.x=searchLight.radius;

  }

    if (searchLight.x+searchLight.radius>=canvasWidth) {

                   searchLight.vx=-searchLight.vx;

                   searchLight.x=canvasWidth-searchLight.radius;

    }

    if(searchLight.y-searchLight.radius<=0){

    searchLight.vy=-searchLight.vy;

    searchLight.y=searchLight.radius;

    }

    if (searchLight.y+searchLight.radius>=canvasHeight) {

                   searchLight.vy=-searchLight.vy;

                   searchLight.y=canvasHeight-searchLight.radius;

    }

}

</script>

我把完整的给你，你自己对下哪里不同。