The purpose of this research is to generate a stable motion for a hydraulic actuated biped robot. Since the application effect of most dynamic biped robot locomotion theories is not very well and the static walk pattern is hard to realize on human-sized hydraulic actuated biped robot because of the small size of foot compared with body height. In this study, we propose a trajectory planning method based on static walking strategy. Firstly, the mechanical structure and kinematics model of the hydraulic biped robot are described. Then, we analyze why biped robot always falls backward during the walking period and propose an improved motion by adding a section of CoG movement during single support phase. The gait planning is realized with cubic spline trajectory. Finally the motion is verified with coordinated simulations based on ADAMS and MATLAB software.