The present invention provides a back frame of flat panel display device and a backlight system thereof, which include at least two primary assembling pieces, and the at least two primary assembling pieces are joined to form a main frame structure of the back frame. The back frame further includes a cable protection structure and the cable protection structure receives a flexible circuit board or a cable. The back frame mold of the back frame and the backlight system thereof of the present invention have a back frame mold that is of a simple structure, reduce the expenditure of the back frame mold, save the material for the back frame, lower down the cost of flat panel display device, and offers an advantage of protecting flexible circuit board or cable.