The present invention provides a back frame of flat panel display device and a backlight system. The back frame includes primary assembling pieces, secondary assembling pieces, and a bracing piece for fixing a circuit board. The primary assembling pieces are connected through joining. Further, the primary assembling piece includes a joint section that forms a reinforcement structure and a circuit board is mountable and position adjustable through a bracing piece. The present invention also provides a backlight system. The back frame and the backlight system of the present invention have a back frame of simple structure, reduce the expenditure of a back frame mold, allow strength of the back frame to meet a desired requirement, improve fixing of circuit board, and save the material used for back frame so as to lower down the manufacturing cost of flat panel display device.