A type of pump reflector for large aperture active mirror slab amplifier is designed. The amplifier is different from the conventional multi-segment-amplifier (MSA). The reflector should be specially designed to improve the efficiency of the pump Xe-lights. For the unique structure, the theoretical optimized curve function is calculated by Matlab. TracePro is used to simulate the designed reflector based on the optimized function, and get the theoretical efficiency of the reflector. Compared to the flat reflector, the efficiency is 17.58% higher. A 11 experiment is designed and used to test the performance of the reflector, whose efficiency is 15.19% higher than the flat one.