The present invention provides a back frame of flat panel display device, a mold for back frame, and a backlight system. The back frame includes at least two primary assembling pieces and the two primary assembling pieces are joined to form a main frame structure of the back frame. The at least two primary assembling pieces that are in direct contact for joining or indirect joining to form the main frame structure of the back frame are set in mutual rotation symmetry of a predetermined angle with respect to each other. The back frame, the mold for back frame, and the backlight system of the present invention, through providing at least two primary assembling pieces and the at least two primary assembling pieces being set in mutual rotation symmetry of a predetermined angle with respect to each other, make it possible for one set of mold to be shared by various primary assembling pieces so as to reduce the cost of mold and thus lowering the manufacturing costs of a backlight system and a back frame thereof.