const FILE_PATH:String="main.swf";
const CLASS_NAME:String="MainSwf";

var loader:Loader;
var request:URLRequest;

loader = new Loader();
request=new URLRequest(FILE_PATH);
loader.contentLoaderInfo.addEventListener(Event.COMPLETE,onComplete);
loader.contentLoaderInfo.addEventListener(ProgressEvent.PROGRESS,onProgress);
loader.load(request);

function onComplete(e:Event):void {	
	var domain:ApplicationDomain=e.target.applicationDomain as ApplicationDomain;
	var swfClass:Class=domain.getDefinition(CLASS_NAME) as Class;
	var swfInstance:Sprite = (new swfClass()) as Sprite;
	swfInstance.x=0;
	swfInstance.y=0;

	//卸掉舞台上现有的东西	
	var _childCount = numChildren;
	for (var i:int=_childCount-1; i>=0; i--) {		
		this.removeChildAt(i);	
	}	

	//加载刚下载的动画
	this.addChild(swfInstance);
}

function onProgress(e:ProgressEvent) {
	//trace(e);
	label1.text = "正在加载，" + e.bytesLoaded + " / " + e.bytesTotal + "， " + (Math.round(  (100 * e.bytesLoaded / e.bytesTotal) / 0.01) * 0.01) + " %";
}

stop();

解释:as3中每个fla都可以对应一个.as文件，用于实现代码界面分离，上面代码中的CLASS_NAME即为.as文件中定义的类名

注意：如果在Main.fla中用到了系统组件(比如TileList之类)，最好在Loading.fla中也拖一个出来到舞台上，然后删除，否则在loading中加载main的实例时，会出一些莫名其妙的问题!