A flat light source module including a light guide plate, a flexible circuit board and a light emitting device is provided. The light guide plate has a light incident surface, a light exit surface and a bottom surface opposite to the light exit surface, wherein the light incident surface further includes a light incident curved surface, and the light incident curved surface is connected with the light exit surface and the bottom surface. The flexible circuit board is disposed beside the light incident curved surface along the light incident curved surface of the light guide plate. The light emitting device is disposed on the flexible circuit board, and the light emitting device has a light emitting surface facing the light incident curved surface.