A light source module includes a ring light guide, at least one light source, a reflector, and a diffuser. The ring light guide includes a ring body and at least one light incident portion. The ring light guide has a light exit surface and a bottom surface, which are two opposite ring surfaces of the ring body. The ring body has a plurality of optical structures disposed along the ring body. The at least one light incident portion is disposed on the ring body and protrudes from a lateral side of the ring body. The light incident portion has a light incident surface, wherein a plane where the light incident surface lies crosses a plane where the light exit surface lies.