A backlight module assembling apparatus for attaching a reflector plate to a driving circuit board is disclosed. The backlight module assembling apparatus includes an assembling portion and a pressing movably connected to the assembling portion. The assembling portion defines a receiving space configured for receiving and positioning a driving circuit board therein. The assembling portion includes a top plate and a first opening defined in the top plate. The first opening communicates with the receiving space. The assembling portion includes a number of blades arranged around the first opening for cutting a reflector plate. The pressing portion includes a pressing plate. The pressing plate is movable toward and away from the blade and configured for pressing the reflector plate against the blades thus allowing the blades cutting the reflector plate to a shape conforming to the first opening and attaching the reflector plate to the driving circuit board.