The parallel mechanism composition theory and the design method for topology structure of the parallel mechanisms were studied based on the basic kinematic chain (BKC) units. Firstly, a new expression for the parallel mechanism composition theory based on BKC was proposed, which had clear physical meaning. Secondly, according to the composition theory and the formula for calculating the value of the coupling degree k , 53 kinds of BKC with coupling degree k =0~3, the basic loop v =1~5, as well as their analytical expressions were presented. It was found that there were six mostly common types of basic BKCs and these basic BKCs can constitute 40 kinds of BKC topology structure which are the basic unit of parallel mechanism. A concept of virtual POC set of BKC topology structure was proposed and 40 kinds of BKC topology structures were classified according to 13 kinds of virtual POC sets with the coupling degree k , which provided the composition unit, i.e., building block of a parallel mechanism, for designing the topology structure of parallel mechanisms. Finally, a general and practical method for topology structure design of parallel mechanism was presented with BKC as the composition units, and meeting the position and orientation characteristics (POC) set as the design goal. Three examples were illustrated. The method had only three steps and no need to synthesize the branch chains and their topology structures between the base and the moving platform. Compared with the existing four main methods for type synthesis, this new method has better usability and universality.