Common platform design for product families is an important step in the modularity field. It is impossible to find the similar components in alterative design stages for the traditional similarity measure used in module division. So, a new common platform design method on extension-distance is presented. In different design environments, three distance concepts are discussed, namely, feature-to-feature, feature-to-set and set-to-set. The first concept will be employed as the base for extension-cluster method in the platform design on the thought of each structure’ s features between two adjacent structure components, while the second is used to build the tolerance-constraint function in the customized components division for the common platform generation. Last the adaptability measure is used to find the optimal case in the database. The method took the gasoline saw as an example.