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 to form a main frame structure of the back frame. The secondary assembling pieces are joined to the primary assembling pieces. According to practical needs of the main frame structure, different materials are used for different parts at different locations. The back frame is provided with the bracing piece for fixing a circuit board, and the bracing piece includes a bracing body, a first suspension section, a first bearing section, and a first resilient bent section. The present invention also provides a backlight system. The back frame and the backlight system of the present invention have a back frame having a simple structure, overcome the problem that a circuit cannot be easily fixed, 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.