Abstract: In the weightlessness environment of the space, the dynamic model of the space manipulator for the on-orbit assembly system is different from that under the ground condition. In this paper, with the spatial 3-DOF manipulator as the subject, the dynamic equation is derived by using the Lagrange equation tailored to the space environment. The dynamic response of each joint is obtained by the simulation of different critical moments, and the accuracy of the dynamic model is verified by the simulation results. The coupling of different rotation angles due to different joint torques is clearly shown. At the same time, a new trajectory approximation strategy is developed on the basis of the traditional path planning method. Comparison of the two methods shows that the point-to-point approximation method enjoys a better precision and a higher efficiency, allowing the engineering task to be carried out according to the desired path in the space environment. The above studies can also be used for the design of the space manipulator system.