Expounds characteristics of the frame structure and function of a hydraulic walk caterpillar vehicles. A frame structure is designed to suit for a caterpillar running system with hydraulic pump- motor circuit. While to design such a frame, there must be a tradeoff between mechanical property and its mass. Based on establishing the geometric model of the frame, the finite element method is used to analyse the distribution of stress and strain of the frame in its working condition. Furthermore, aim to reduce the mass of the frame, a programme is designed and applied to find the best tradeoff point between requirment and mass. The programme is developed using APDL(ANSYS Parametric Design Language). The simulated example showed that the propose method in this paper can be applied to optimize the frame of caterpillar vehicles.