A foolproof frame assembly includes a blocking component fixed on a case, and first and second frames disposed on opposite sides of the case. Each of the first and second frames has a foolproof component fixed to one of two side plates thereof and blocking one of two through holes on its bottom plate. When the first frame is in a first mounting position, the blocking component passes through another through hole and form a space with another side plate of the first frame for one of two mounting side plates of the mounting frame to be disposed therethrough. When the second frame is in the first mounting position, a space occupied by its foolproof component overlaps a space occupied by the blocking component. When the first and second frames are respectively in the first and second mounting positions, the foolproof components block the two mounting side plates.