Abstract: In this article, We have analyzed the coupling of passive joints and actuated joints in passive redundancy space robots, and formulated a measure about the coupling which can be considered as a performance index in robot motion planning. We also analyzed the kinematics singularity of passive redundancy robots and the difference about the singularity between the passive and the non-passive redundancy robots. We defined a passive redundancy robot manipulability index which also can be used in robot motion planning and presented the concept of "proximate self-motion" for the passive redundancy robot, the "proximate self-motion" can optimize the motion when robot completes a end-effector mission. In the simulation faction, a planar 3-DOF space station robot exhibits the ability of the "proximate self-motion" in avoiding the kinematics singularity.