The present invention provides a back frame of flat panel display device, which includes primary assembling pieces, secondary assembling pieces, and a bracing piece for fixing a circuit board. The primary assembling pieces have a number of at least two, and the at least two primary assembling pieces are connected through joining. The secondary assembling pieces are joined to the primary assembling pieces. The bracing piece includes a bracing body, a first suspension section, a first bearing section, and a first resilient bent section. The bracing body is mounted to the primary assembling pieces or the secondary assembling pieces or is mounted to both the primary assembling pieces and the secondary assembling pieces. The available mounting points between the bracing body and the primary assembling pieces or the secondary assembling pieces are of a number of at least two. The bracing piece fixes a circuit board. The present invention also provides a backlight system. The back frame and the backlight system of the present invention have a back frame mold having a simple structure, reduce the expenditure of a back frame mold, and save the material used for back frame so as to lower down the cost of flat panel display device.