One kind of LED backlight module used for large-size TFT-LCD to apply in the direct illumination-type backlight source is designed by theoretical analysis and software simulation, and it is also confirmed such 15-inch backlight source can provide uniform illumination. The LED light intensity is estimated according to backlight module surface's brightness. A LED illumination model in Tracepro is established to study its illumination distribution. It is found the illuminance is most uniform when the mixed cavity thickness is 20mm and the backlight source LED spacing 20mm. Simultaneously the promotion to the LED backlight source array, its optics uniformity achieves 85%. By adding a lens for each LED to design those according to the Tracepro interactive optimized tool, the mixed light space can be reduced to 5-10mm.