flash的AS2.0中如何检测键盘是否按下某键？ AS3.0中怎么检测呢？

2.0的没兴趣，3.0的我帮你一段代码，可以直接测试：
var child:Sprite = new Sprite();
var bgColor:uint = 0x00CCFF;
var size:uint = 80;
child.graphics.beginFill(bgColor);
child.graphics.drawRect(0, 0, size, size);
child.graphics.endFill();
addChild(child);
child.addEventListener(MouseEvent.CLICK, clickHandler);
child.addEventListener(KeyboardEvent.KEY_DOWN, keyDownHandler);
child.addEventListener(KeyboardEvent.KEY_UP, keyUpHandler);

function clickHandler(event:MouseEvent):void {
stage.focus = child;
}
function keyDownHandler(event:KeyboardEvent):void {
trace("keyDownHandler: " + event.keyCode);
trace("ctrlKey: " + event.ctrlKey);
trace("keyLocation: " + event.keyLocation);
trace("shiftKey: " + event.shiftKey);
trace("altKey: " + event.altKey);
}
function keyUpHandler(event:KeyboardEvent):void {
trace("keyUpHandler: " + event.keyCode);
}

如图框起来的，当左键按下时