This paper designs a hydraulic-actuated biped robot including the aspects of mechanical structure and gait planning. Referring to skeleton structure of human, we designed a 3D biped robot model. The robot has 12 degrees of freedom. Each freedom covers the range of motion in human walking. Based on the loaded inverted pendulum model, stable walking gait is planned. Mathematical relationship between joint trajectory and walking parameters is obtained. Finally, walking simulation is made to verify the feasibility of gait planning and rationality of mechanical design. ANSYS has been used to check stress in important parts. The force curves of hydraulic piston rodsare also obtained by simulation which provides reference to select hydraulic components and build a physical prototype.