To deal with mass knowledge with contrary precondition in product family configuration planning more efficiently,a product family configuration planning model was proposed based on extension logic,which consisted of customer requirement acquisition,extension transforming and extension cluster to solve the problems of configuration requirement dynamical increase,configuration threshold value determination,and conflict-resolving.Firstly,the basic requirement elements for a series product were confirmed by conjugating analysis,and the mapping relationship between requirement and product family structure feature was constructed.Then by applying extension rule reasoning,a series of product could satisfy the customers' requirements were obtained from qualitative perspective.Secondly,the basic requirement element was transferred by transfer matrix,and it was output again by customer to calculate the tolerance with the initial basic requirement element.Then the transformation feasibility was determined by the configuration threshold value,which was gotten dynamically by improved genetic algorithm.Finally,the stable module and changeable module in product family were defined by using extension cluster and cluster graph.This method was applied in fridge configuration prototype system design.