In order to solve problems concerning the complex module replacement and positioning difficulties in the modular design process, the control-model which reflects the main structure of product is established. Control-model bases itself on the product’s absolute coordinate system, and by establishing datum plane and sketches it reflects the main structure of products. According to the relationship between modules, modules can be partitioned into main-modules and attached-modules. The main- and attached-modules established by using control-model as seed file contain relevant information of the control-model, therefore the method of modules’ rapid positioning by comparison and updating as well as module change strategies can be used to realize modular design process without assembly constraints. Finally, taking a certain type of tank semi-trailer as an example, this paper makes detailed explanation about the modular design method based on control-model.