A liquid crystal display backlight module controlling method and a controlling device thereof disclosed in the present invention includes a programmable logic gate array board extracting backlight information from an image signal. Calculation of local dimming is performed by the programmable logic gate array board, a calculation buffer for local dimming calculation is sufficient, which can increase liquid crystal display backlight divisions. Furthermore, the present invention employs a row signal and a column signal to control and switch on light emitting diodes in the backlight light board, which can achieve the cost lowering effect.