实现的代码如下：

var distance:Number = 10;
this.createTextField("display_txt", 999, 0, 0, 100, 20);
var keyListener:Object = new Object();
keyListener.onKeyDown = function() {
 if (Key.isDown(Key.LEFT)) {
  car_mc._x = Math.max(car_mc._x - distance, 0);
  display_txt.text = "Left";
 } else if (Key.isDown(Key.RIGHT)) {
  car_mc._x = Math.min(car_mc._x + distance, Stage.width - car_mc._width);
  display_txt.text = "Right";
 } else if (Key.isDown(Key.UP)) {
  car_mc._y = Math.max(car_mc._y - distance, 0);
  display_txt.text = "Up";
 } else if (Key.isDown(Key.DOWN)) {
  car_mc._y = Math.min(car_mc._y + distance, Stage.height - car_mc._height);
  display_txt.text = "Down";
 }
};
Key.addListener(keyListener);

注：该段代码最有趣的是边界检测部分的代码，可以算是边界检测时的一种新思路吧。