To improve the quality and energy efficiency of road lighting, the reverse optimization design method is employed to design a light-emitting diode (LED) street lamp lens. By taking the road lighting standard as constraint, an ideal light intensity distribution with maximum brightness illumination ratio is optimized and obtained. Based on the optimized ideal light intensity distribution, the freeform surface is established by using the variable separation mapping method. The light source and target plane are meshed to form the mapping relationship, and the surface structure error is controlled as well. Then the lens is optimized by using feedback optimization to make the intensity distribution of lens close to ideal light intensity distribution. Simulation experiment results show that the lens designed by the proposed method conforms to the road lighting standard well, and the brightness illumination ratio is 0.067. Compared with traditional symmetry light intensity distribution, energy consumption is reduced to 20.2% . The high quality lighting and energy utilization ratio of LED street lamp are realized finally.