Abstract: The hydraulic legged robot has been widely concerned because of its strong load capacity and excellent dynamic performance.In this paper,the hydraulic robot leg was investigated based on the spring-loaded inverted pendulum (SLIP) model to solve the energy regulation and hopping control problem for hydraulic robot.And an active variable stiffness control strategy was proposed to adjust the energy of hydraulic robot leg in the stance phase and ensure its desired hopping height.The experimental results show that the control strategy proposed in this work can achieve a stable hopping for the hydraulic foot robot leg and control its hopping height effectively.