According to the design principles and structure characters of the plane porthole extrusion moulds of hollow aluminum profiles, the general structure of a UG based CAD system for porthole extrusion moulds of aluminum profiles is designed after system analysis and plan, by adopting the object oriented technology and OMT (Object Modeling Technology) methods. Then the CAD system is developed based on UG software with the help of UG / Open GRIP re-developing tools. The sample design approved that it is quite convenient to design parametric porthole extrusion moulds CAD model by using the developed CAD system. In the last part, the FVM (Finite Volume Method) is used to simulate the process of extrusion. The metal flow, the equivalent stress distribution are analyzed for future optimization of the porthole structure.