The present invention discloses a flat display device and a manufacture method thereof. The flat display device includes a flat display module, a front cover, an auxiliary support, a back cover set, and a circuit board. The back cover set includes a sub-cover and a main back cover, wherein the circuit board is disposed on the inner surface of the sub-cover. The front cover has a display opening for an active area of the flat display module to be exposed outside the display opening and present images through the display opening. The main back cover includes a opening for part of the sub-cover to pass through and be exposed outside the opening.