The parallel mechanism with position and orientation characteristics limb (POL) is a kind of special low mobility mechanism which consists of 2~5 identical active unconstrained limbs and a passive POL. The motion characteristics of the mechanism are determined by POL, thus it was referred to as motion constraint generator by some literatures. This parallel mechanism with POL has many excellent characteristics, such as high rigidity, high control accuracy and bearing capacity, and the synthesis process is simpler than the other mechanism. Firstly, principle of composition of parallel mechanism with POL was proposed, and some formula of topological structure characteristics were simplified, including its position and orientation characteristics equation (POC), degree of freedom (DOF) and coupling degree, meanwhile, synthesis process of topological structure was listed. Secondly, topological structures of POL with 2~5 DOF and unconstrained active limbs were synthesized according to POC theory. Type synthesis of 4-DOF parallel mechanism was used as an example to illuminate the principles and methods of designing this kind of mechanism. In this section, the concept of generalized POL is mentioned, which contained not only traditional precisely constrained passive chains, but also some active branched chains. Thirdly, the mechanism was classified on the basis of the number of driving pairs in POL, and several cases were designed of each type. Lastly, two ways of structure optimization for chain structure and mechanism symmetric were presented to improve mechanical properties of partial examples. The research provided a more systemic theory and design methodology for topological structure synthesis of the parallel mechanism with POL than previous literatures.