Along with a development of the industry, wearable devices are spreading. These devices often have a problem of a battery exhaustion. Biomechanical energy harvesting is one of the methods to solve this problem. As methods of biomechanical energy harvesting, the shoes which attached piezoelectric elements and the system which attached a generator to the knee were researched.This paper focuses on the surplus energy when walking down a slope. In order to determine the amout of the surplus energy, a passive walking model is utilized. Additionally, a method to recover surplus energy while maintaining walking stability is introduced. Finally, PI control of the boost chopper is utilized to adjust a braking torque and the amount of the generation. The validity of the proposed method is confirmed by simulation.