Accurate measurement of physical properties of LED light source and an appropriate modeling are crucial for LED optical system design and computer simulation. The characteristics of common light source model are analysed. A comparative analysis of the design and simulation results for an actual optical system is given caused by the deviation of actual LED light source characteristics from that of an ideal point light source and simple plane surface light source. A feedback method of lens design consisted of internal and external refractive surface is presented for a real direct- type backlight system with a white LED light source luminance data. Simulation results show that the uniformity of illumination reaches 0.83 when the distance-height ratio is 5.5 and the source area is near 1.7 mm×1.7 mm.