A light guiding plate structure comprises an outlet surface; a first side surface and a second side surface both of which are connected to the two sides of the outlet surfaces respectively; and a bottom surface connected to the first side surface and a second side surface. The bottom surface comprises the inlet surface and a bottom reflective surface, the upright projection of the first side surface on the bottom surface contains the inlet surface, light ray emitted from the inlet surface is reflected by the first side surface to one of the second side surface and the bottom reflective surface, and is emitted by the outlet surface. The first side surface comprises a third location surface connected with the outlet surface, a first inclination surface connected with the bottom surface and the third location surface. The invention also provides a backlight module comprising the light guiding plate structure. Light ray emitted by the light source can be irradiated evenly by the light guiding plate to improve the integral brightness evenness and solve the problem of uneven brightness caused by the thin type of the backlight module. In addition, the number of the light sources is reduced, so that weight of the backlight module is lowered.