A covering assembly for a building opening is disclosed, which includes a first frame and a second frame with one in front of the other. The first frame has a first shield structure provided thereon to shade or open a surrounded opening of the first frame, and the second frame has a second shield structure provided thereon to shade or open a surrounded opening of the second frame. The first frame further has a first positioning structure, and the second frame has a second positioning structure, wherein the second positioning structure and the first positioning structure are mortised together, so that the first frame and the second frame could be quickly aligned with each other and mortised together while installing.