The paper illustrates the function, UI(User Interface), structure and key technology of the Extruder CAD system, which is based on Solidworks. The whole system is encapsulated in a menu of Solidworks. And after some parameters, such as power and rotational speed are imported through UI(User Interface), the system will automatically complete the design of extruder, including the whole process of design of the extruder case, system of pulley transmission and reducer transmission. Then 3D models and engineering drawings of parts or assembly can be exported according to the choice of user. The realization of the automatically 3D modeling adopted the idea of being driven by dimension primarily and being driven by program secondarily. Visual C develops the program of the system, and it can be compatibly added to Solidworks as dll (dynamic linkage library). Combined with functions of 3D modelling, design and assembly of Solidworks, the efficiency, precision and quality of design of extruder are greatly improved.