The invention, which relates to the technical field of the robot, provides a calculation method of a kinetic model of a 3D under-actuated biped robot. For a 3D under-actuated biped robot, kinetic model calculation is carried out on a one-foot supporting phase and a biped supporting phase, wherein the biped supporting phase of the robot is formed by a collision process and a coordinate switching process. Compared with the previous kinetic model calculation method, the provided method enables the programming calculation process of the kinetic model of the 3D under-actuated biped robot to become simple and clear, so that programming application can be realized conveniently.