In view of the need for secondary optical design of LED as the stage light, a design for reflecting stage light with high-power LED is proposed to achieve the asymmetric uniform illumination on stage backdrop, according to the principle of extended light source. By creating a reflecting surface model in a two-dimensional coordinate system and controlling the two marginal rays of a certain point on the target surface, the light cone formed by the two marginal rays can sweep point by point. Combined with the laws of reflection, the edge ray principle and the étendue conservation theory, the coordinates of each point on the generatrix are obtained and then the reflection surface is generated after stretching the generatrix. The ray-tracing simulations with TracePro software show that by superposition of several stage lights, in the illuminated region of 20 m width and 11 m height within a distance range of 1.5~2.0 m, the luminous uniformity is able to reach 80%. Hence, this structure enables LED light source to be applied in wide-range lighting system to get uniform illumination.