In order to improve the efficiency of human transport，reduce the joint damage and improve the energy utilization efficiency of the exoskeleton，the stiffness of an energy storage element of the lower extremity exoskeletons was optimized based on manmachine dynamics. The Newton-Euler dynamics equation was used to establish a man-machine coupling dynamics model. The mathematical models of the relationship between the joint torque and the length，mass，joint angle and spring stiffness were obtained. The Angle data collected by the 3D motion capture system was substituted into the dynamic equation，and the dynamic variation rule of joint torque was calculated by MATLAB. The energy flow model of each part of the man-machine system was established，and the energy flow analysis in the gait cycle was carried out. The stiffness of the energy storage element was taken as the parameter，the energy flow of the energy storage element was taken as the constraint condition，and the minimum torque of each joint was taken as the goal to establish the optimization model. By comparing with the human model obtained by AnyBody human simulation software，the correctness of the optimization results was verified. The results showed that the wearable exoskeleton can reduce the impact of lower limb landing on human body，effectively reduce the energy consumption and the lower limb joint torque.