The present invention relates to a backlight of a liquid crystal display device and a driving method thereof. The backlight comprises a LED lamp bead group and a backlight driving module. The LED lamp bead group includes a plurality of strings of LED lamp bead which are opposite in term of positive and negative directions. The backlight driving module is electrically connected to the LED lamp bead group to output a voltage signal and a current signal to drive the LED lamp bead group, wherein the voltage signal is configured as more than one serials of periodic square-wave signal, under the action of which a string of LED lamp bead of positive direction and a string of LED lamp bead of negative direction emit light in an alternate manner, and the brightness of emitted light is determined by the magnitude of the current signal.