A back frame and a backlight system of a flat panel display device are disclosed. The back frame includes at least two assembling pieces joined to form the back frame and a bracing piece fixed to the assembling pieces. The assembling pieces are mountable to at least two different positions of the bracing piece in a lengthwise direction of the bracing piece. By making the mounting position of a bracing piece to an assembling piece adjustable in a lengthwise direction of the bracing piece, a single bracing piece may meet different needs for mounting so as to reduce the manufacturing cost of the flat panel display device.