import flash.display.StageAlign;
import flash.display.StageScaleMode;
import flash.events.Event;

function resizeHandler(e:Event):void
{
  yourMC.x = (yourMC.stage.stageWidth / 2) - (yourMC.width / 2);
  yourMC.y = (yourMC.stage.stageHeight / 2) - (yourMC.height / 2);
  //如果想要有动画效果，在此加..
  //用 Tween 类即可实现
}

stage.align = StageAlign.TOP_LEFT;
stage.scaleMode = StageScaleMode.NO_SCALE;
stage.addEventListener(Event.RESIZE, resizeHandler);
stage.dispatchEvent(new Event(Event.RESIZE));