A light source module includes at least a light source and a housing. The housing includes a base having a slanted reflective surface, a plurality of sidewalls extending out of a peripheral of the base cooperatively defining an opening with the base, the sidewall aligned with a trough of the slanted reflective surface having an inner surface and an outer surface opposite to the inner surface, and a plurality of fin structures formed on the outer surface of the sidewall. The light source is fixed on the inner surface of the sidewall. Light rays emitted from the light source being reflected at the slanted reflective surface toward the opening. A backlight system using the light source module is also provided. The present backlight system has a good heat dissipation capability due to an employment of the present light source module, and can be configured to be a thin body.