The present invention discloses a side-light type light guide plate assembly and a backlight module, which has a light guide plate and at least one reflector sheet. The light guide plate has a lower surface, a light output surface opposite to the lower surface, and at least one light input side surface connected between the lower surface and the light output surface. The reflector sheet is correspondingly disposed on at least one side surface of the light guide plate except for the light input side surface, while a medium layer is defined between the reflector sheet and the light guide plate. The refraction index of the medium layer is smaller than or equal to 1.12. By providing the air layer and the reflector sheet, light beam emitting out of a side of the light guide plate and going back to the light guide plate can meet conditions of total reflection, so as to prevent from generating a light leakage phenomenon at edges of the light guide plate.