This thesis presents a LED backlight driving system suitable for large-size LCD device with direct type LED backlight. The DC/DC conversion uses a high step-up converter for reducing the required LED light bar sets; The LED dimming driving circuit uses a digital control IC to drive for saving the required components of the dimming control circuit; The dimming control employs local dimming strategy to reduce the unnecessary waste of energy. Finally, a LED backlight driving system has been implemented and tested to verify its functions, the experimental results show that the circuit's conversion efficiency can be reach above 92%.