Abstract: The kinematics of a three-branch space robot mechanism was analyzed by presenting its model and discussing its three working modes. The space robot is a kind of serial-parallel mechanism when it is working in carrying and cooperating modes. Since six out of its twelve joints are independent in this case, it is difficult to analyze and control this robot. By using influence coefficient method, the motion constraint equations of the mechanism were derived. The mapping of input and output was established. The inverse kinematics about velocity and acceleration were also solved.