I figured it out:D. if not misunderstanding:
1.set global variable CurrentLevel = layoutname in level layout. set global variable IsHUDLoaded = 0.
2. if IsHUDLoaded = 0, goto HUD layout.
3. in HUD layout, set IsHUDLoaded = 1. then goto layout by name CurrentLevel.
but there is one problem left: if I quit to Main menu, which is a simple layout when game start. the HUD objects are there too. how do I remove them?