A light source module includes a casing, at least one reflective layer, a light bar, and at least one light source. The casing has a first side wall and a second side wall, and the first side wall and the second side wall define an opening. The opening faces a light incident surface of a light guide plate, and the first side wall and the second side wall are respectively positioned on two opposite sides of the opening. The reflective layer is formed inside the casing and adjacent to one of the first side wall and the second side wall. The light bar is disposed inside the casing and forms an angle with the light incident surface of the light guide plate.