The structure size and drive forms of walking assist robots hip joint, knee joints and ankle joints are determined and the assembly of these parts are also completed through the analysis of human gait characteristics. The Lagrangian algorithm was used to complete the robot kinematics analysis, and the action of the hydraulic cylinder was optimized in algorithm to design the schematic diagrams of the drive hydraulic system. At last, using the hydraulic mechanic joint simulation of Virtual. Lab Motion and AMESim, the simulation curve with the actual calculation curve were compared, which confirms the correctness of the kinematic analysis and the feasibility of hydraulic drive system.