As one of China’s traditional martial arts sport, Sanda has been arresting the attention of various countries and regions. Through research, this paper obtains the mechanics relationship, dynamics relationship and rotary inertia of athletes doing rear side kick and learns about the potential energy generated by athletes doing side kick. In the side kick model of Sanda athletes established in this paper, athletes can increase the momentum of knee joint through the effective braking effect of hip joint in accordance with the principle that momentum is transferred from proximal to distal. Then the momentum is transferred to the ankle joint via the braking of knee, resulting in the end acceleration. This paper also draws the conclusion that the angular velocity of the left thigh of athletes doing side kick depends on the angular velocity of the hip joint and the velocity of waist, but that of the calf is determined by the angular velocity of the knee joint and left thigh. In addition, the angular velocity of the calf should be greater than that of the thigh, which is more conducive to the acceleration of the knee joint. When doing side kick, athletes should fully fold thigh and calf and reduce the limb’s rotary inertia to improve the angular velocity of the knee rapidly. In the meanwhile, muscle groups in side kick can be fully contracted to enable athletes’ leg muscle force to achieve the best condition and enhance its contraction speed. China’s athletes can conduct targeted training based on the research result of this paper.