import flash.display.NativeWindowInitOptions;
import flash.display.NativeWindowSystemChrome;
import flash.display.NativeWindowType;
import flash.display.NativeWindow;
import flash.display.StageAlign;
import flash.display.StageScaleMode;
import flash.geom.Rectangle;

var windowOptions:NativeWindowInitOptions = new NativeWindowInitOptions();
windowOptions.systemChrome = NativeWindowSystemChrome.STANDARD;
windowOptions.type = NativeWindowType.NORMAL;

var newWindow:NativeWindow = new NativeWindow(windowOptions);
newWindow.stage.scaleMode = StageScaleMode.NO_SCALE;
newWindow.stage.align = StageAlign.TOP_LEFT;
newWindow.bounds = new Rectangle(200, 200, 800, 800);

newWindow.activate();

参考:https://help.adobe.com/zh_CN/FlashPlatform/reference/actionscript/3/flash/display/NativeWindow.html#NativeWindow()