折腾了一上午，整理出这两个方法来。

//方法一(推荐)：MovieClipLoader方法

var mcContainer:MovieClip = this.createEmptyMovieClip("myMC", this.getNextHighestDepth());
var mcLoader:MovieClipLoader = new MovieClipLoader();
var mcListener:Object = new Object();
mcListener.onLoadProgress = function(target:MovieClip, bytesLoaded:Number, bytesTotal:Number) {
trace(Math.floor((bytesLoaded/bytesTotal)*100)+"%");
}
mcListener.onLoadInit = function(target:MovieClip):Void {
trace(target._name + " loadedComplete");
}
mcLoader.addListener(mcListener);
mcLoader.loadClip("picImg/pic1.jpg", "myMC");

//方法二：onEnterFrame方法

this.createEmptyMovieClip("tempMC", this.getNextHighestDepth());
this.createEmptyMovieClip("loaderMC", this.getNextHighestDepth());
loaderMC.loadMovie("picImg/pic1.jpg");
tempMC.onEnterFrame = function() {
var loaded = loaderMC.getBytesLoaded();
var loadTotal = loaderMC.getBytesTotal();
trace(int((loaded/loadTotal)*100)+"%");
//此处很奇异，判断是否加载完要用((loaded/loadTotal) == 1)而不能用(loaded == loadTotal)
if((loaded/loadTotal) == 1){
delete this.onEnterFrame;
trace("loadedComplete");
}
}