Edit in JSFiddle

var canvas = document.getElementById("myCanvas");
if (!canvas.getContext) {
    return;
}

var ctx = canvas.getContext("2d");
createRectShadow();

function createRectShadow(){
    ctx.save();
    ctx.shadowOffsetX = 8;
    ctx.shadowOffsetY = 8;
    ctx.shadowBlur = 10;
    ctx.shadowColor = "rgba(0, 0, 0, 0.3)";
    ctx.fillStyle = "#CCC";
    ctx.fillRect(10, 10, 400, 260);
    ctx.restore();
}

<canvas id="myCanvas" width="440" height="300">
    您的浏览器不支持canvas!
</canvas>