Based on the bi-directional approach, the paper discusses the motion planning of freeflying dual-arm space robot systems. With the linear and angular momentum conversation, the state equation is established for the system control design. And then applying the Lyapunov method to the state equation, the control input equation of the manipulator joint angles controlling the vehicle attitude and the manipulator joint angles is obtained. Because no continuous trajectory from the start point to the end point in the state space can complete the desired task, the synthesized trajectory is gained to accomplish the desired task through the bi-directional approach. The advantage of the approach is to decrease the fuel consumption for controlling the vehicle attitude and prolong the available life-span of space robot systems. A planar dual-arm space robot system is simulated to verify the proposed approach.