The cat is flexible and skillful in jumping. The analysis of its jumping law can provide inspiration for the design of leg jumping robot. In this paper, a cat-like robot which can simulate cat jumping is used as a model to simplify its structure and analyze its dynamics and kinematics, and the law of the waist moment changing with time is obtained, so as to analyze the role of the waist in the take-off process. From the energy point of view, the relationship between the initial waist angle during take-off and the work required to reach a specific take-off posture mechanism is analyzed, and the difference between the two is compared. The analysis results show that the waist posture adjustment in the take-off preparation stage can improve the efficiency. It proves that the mechanism with waist-assisted jumping is more flexible and efficient than the mechanism without waist-assisted jumping.