Abstract: An optical lens with big angle is developed for the uniform illumination system of LED direct backlight based on two free surface. The light of LED is firstly diverged by near free surface with the light distribution of c/cos³(θ) type. Then, the uniform illumination of target is achieved by far free surface. With this design, the uniform illumination is achieved on short distance, the angle of total reflection is avoided and the area of lighting is enlarged in compared with the design of single free surface. The structure is simulated by ray tracing software. The triangle LED layout from the result of simulation, the area of LED light reaches 60 mm in diameter on condition of single lens and the target area reaches 87.5% uniform degree with triangle LED layout in only 15 mm height of backlight box. So, in compare with traditional direct structure of high power LED, the uniform degree of lighting is enhanced and the height of back light box is decreased by using the structure of this lens for high power LED.