A light source module including a light guide plate, a light-collecting structure and a light emitting device is provided. The light guide plate has a first light emitting surface, a bottom surface, a first light incident surface connecting with the bottom surface and the first light emitting surface, and an optical microstructure unit. The optical microstructure unit includes at least two optical microstructures. Each optical microstructure has a recessing part and a protruding part on the bottom surface. The light-collecting structure has a second light emitting surface connecting with the first light incident surface, a second light incident surface and a reflecting surface connecting with the second light emitting surface and the second light incident surface. A sectional line obtained by sectioning the reflecting surface along a first reference plane includes a first parabola. The reference plane is parallel to the first light emitting surface.