Accurate acquisition and expression of customer requirement information is the primary step of product family planning under mass customization. Aiming at requirement information with the characteristics of abstractness and vagueness in the process of constructing requirement model, the descriptive demand model of the customers was built on the basis of axiomatic design. The fuzzy demands were quantified through conjoint analysis and transformed into specific performance index and characteristic parameters of product design. Then the design incidence matrix was established through analyzing the correlation between those indexes and parameters, and the matrix was clustered by the fuzzy tree clustering algorithm. According to the clustering validity index, the best clustering thresholds were identified, and the optimal clustering results could be obtained. On this basis, the design association matrix was decoupled and the decoupling design correlation matrix was acquired, which provided the decision guidance for product family planning. Finally, an example of the cylindrical gear reducer product plan was used to demonstrate the feasibility and practical value of the proposed method.