The present invention provides a side-light backlight module with assisting solar light source and which comprises a side-light source; a waveguide having an incident surface facing the side-light source and an emitting surface; an optical film set arranged on top of the waveguide and facing the emitting surface; and an assisting light source directing collected solar light into the incident surface of the waveguide. With the provision of the assisting light source, the solar light can be used as an assisting light source making the backlight module and the liquid crystal display device made therefrom become more environmental friendly.