By combing the technologies of ASP.NET with secondary development of UG CAD system, a digital design system in main reducer based on B/S and C/S hybrid mode has been built. The system has the functions as optimal design of structure parameter, parts of three-dimensional parametric design and virtual assembly. The frame structure and function modules of the system are discussed in detail. The main reducer digital design system has carried on the confirmation through the concrete example. Compared with the traditional design method, this method upgrades the designing ability and efficiency of the enterprise.