The main idea of variant design is to adapt the existing product or module to the new requirements. Based on the analysis of the basic problem solving model of variant design, a case-based reasoning system combined with rule-based reasoning and constraint satisfaction method to imitate the process of variant design is proposed in this paper. With the application——“spindle machine module variant design system” as an example, the knowledge representation model and the reasoning process of the variant-design-oriented cooperative reasoning system are discussed in detail.