On the foundation summing up existing intelligent conceptual design method, this article puts forward the research content, characters, path, and method of the conceptual design of extension for mechanical products. Around the core technology of intelligent conceptual design this article conducts research on the modelization method of extension design in function-principle-layout-configuration. It includes the function expression, function decomposition and synthesis, function illation and decision. Computers are utilized to simulate the human dialectic thought when resolve problems in this method. The living example shows that the extension method has been applied in the field of conceptual design for mechanical products. This method is of great significance to resolve the bottleneck problem in theoretic studying and engineering realization of intelligent CAD.