Based on screw theory, the relationship between motion screw system of limbs and constraint screw system of moving platform were analyzed. All possible 5-order motion screw systems of limbs’ structures were enumerated, and then a systematic method was presented for structural synthesis of 4-DOF (4-degrees-of-freedom) parallel manipulators with symmetrical structure. According to their motionproperty, using this method, a serial of manipulators with 3-translation and 1-rotation mobility that had different structures were gotten. Results show that method has simple expression form for structural synthesis of lower mobility parallel manipulators relying on the relationship between motion and constraints that both have the screw models for easily analyzing. Results also show that the method can be easily applied for structural synthesis, and can be efficiently and fleetly used for structural synthesis of symmetrical mechanisms based on the given motion property of moving platform. It establishes the foundation for optimizing design for lower mobility parallel manipulators.