Using topological structure synthesis method based on position orientation characteristics(POC) set with a unit of single open chain (SOC)，the type synthesis of threerotation (3R) parallel mechanisms is studied．The general steps and concrete examples of the topological structure type synthesis of 3R parallel mechanisms were given. Based on this method, 48 kinds of three rotating output (OT3R) parallel mechanisms were obtained, including some previously designed institutions and some new ones. They are analyzed and classified according to the characteristics of topological structure.