A backlight module and a liquid crystal display device are disclosed. The backlight module comprises a backlight source, a beam splitter and a light beam conversion portion. The beam splitter converts light from the backlight source into a first and second polarization light beam. The light beam conversion portion converts the first polarization light beam into a third polarization light beam through total reflection, and outputs the second and third polarization light beam through a light exit side. Alternatively, the light beam conversion portion converts the first and second polarization light beam into a fourth and fifth polarization light beam through total reflection, and outputs the fourth and fifth polarization light beam through the light exit side. To this end, light from the backlight source is efficiently applied to irradiate a liquid crystal display panel, and the brightness of the liquid crystal display panel significantly improves.