For Artificial Intelligence (AI) with the development of modern science and technology, mechanical product design has become a major object of study scholars. This article is the research (CBR) approach to reasoning: first forming machinery products categories namely functional parameters, size parameters, create an instance of the database for storage. Then according to the design of the user associated with gray requests to search similar instances in the instance matching library, find a few examples of high similarity. Then use the modified immune genetic algorithm instance, improving t he similarity similar instances, close to the ideal product design to user needs. The obtained results are stored in the instance of the library, expand the instance library. Finally, an example of CNC machine tool spindle design platform established, indicating the effectiveness of case-based reasoning approach.