A new method to analyze the structure of parallel robot is presented with reciprocal screw theory.Several limb types are analyzed for the 3-DOF（2R1T）parallel robot.When there is only one pure wrench force in one limb

the design rules and the combinations of revolute and prismatic joints are obtained.Through the discussion of relations between spatial wrench force and DOF of parallel robot

the necessary spatial contribution of wrench forces is investigated.Then the parallel structures with 2R1T DOFs that can be realized in potential industrial applications can be get.By the analysis of 3-RPS parallel manipulator

the mutual effect between wrench forces and DOF is explained.Meanwhile

the method can be used for structure synthesis and optimization design of other kinds of low-DOF parallel robots.