In our study, we use lagrange interpolate polynomial equation to calculate the LED unit illuminance distribution (LUID) in LED backlight module. The illuminance uniformity is 94% and 99% for square and triangle arrangement, respectively. Furthermore, we design and optimize the secondary optical lens base on the property of CSM and the target including illuminance and color uniformity simultaneously. According the simulate process, the condition of square arrangement and DHR is equal to 4, the 83.7% of illuminance uniformity can be achieve, as well as the color uniformity can reduce to 0.0039 which below the requirement of 0.004.