1. A light source module, comprising:a casing, having an internal space;
a light guide plate, disposed in the internal space of the casing, the light guide plate comprising:
a plate body, having a top surface, a bottom surface, and a plurality of side surfaces connected between the top surface and the bottom surface; and
an optical microstructure array, located at the top surface, comprising a plurality of microstructure units;
at least one light emitting element, disposed on the side surfaces of the light guide plate; and
a reflector, disposed on the top surface of the light guide plate, wherein the microstructure units are recessed from the top surface towards the bottom surface and are shaped as inverted square pyramids, a pyramid portion of each of the inverted square pyramids is formed by at least four surfaces, each of the microstructure units comprises two opposite first surfaces and two opposite second surfaces arranged in an alternating manner, and angles included between the two first surfaces and the top surface are identical and angles included between the two second surfaces and the top surface are identical in each of the microstructure units.