Abstract: In order to meet the requirements of high speed running, the flexible spine and the leg structure with high cushion performance for quadruped robot, a flexible spine and hydraulic driving hind limb structure were designed for the running quadruped robot based on hydraulic pressure technology. The spine was designed with variable cross section beam to form flexible in the middle. The leg structure was designed with hip joint and knee joint, 3 DOFs. The hip joint was designed with two active DOFs, roll-DOF and pitching-DOF; the knee joint was only with passive pitching-DOF. The mechanical performance of flexible spine was analyzed. Stiffness characteristics and kinematics of the leg structure were analyzed. And the bound gait of robot was simulated. The simulation results show that, the quadruped robot with the flexible spine and the leg structure with nonlinear stiffness can run stably with higher speed in Bound gait, and the foot contact force is low, verifying the feasibility of the designed spine and the leg structure.