A back cover module and assembling method thereof, a backlight module and assembling method thereof, and a flat panel display device and assembling method thereof are disclosed. The flat panel display device includes the backlight module and a display panel disposed on the backlight module. The backlight module includes the back cover module, a reflector disposed on an inner surface of a back cover body of the back cover module, and a light source module disposed on the reflector. The back cover module includes the back cover body, a first U-shaped plastic frame, and a second U-shaped plastic frame. The first and second U-shaped plastic frames are fixed at four corners of the back cover body. By the present invention, a size of the back cover body can be reduced so as to achieve objectives of decreasing cost and weight of the flat panel display device.