In this paper, a new method is presented for the structure synthesis of Low-DOF symmetrical parallel manipulators based on the theory of screws. A parallel manipulator with three rotational DOFs and one translational DOF is used as an example to illustrate the procedure. All possible force constrained limb that can be used to construct low- DOF parallel manipulators can be enumerated, and the potential parallel mechanisms can be thus enumerated, which is very important for the optimal design of new parallel manipulators. Simulation results show the effectiveness and validity of this method.