Nowadays a general platform has played an important role in the product design process. As the traditional similarity measure used in module division is to the design stage in which the features and set are static, it is impossible to find the similar components in alterative design stages. For this reason, a new common platform design method on extension-distance is presented. According to build the relative function, the threshold used in the key module recognition is easy to generate. Then the tolerance-constraint function in the customized components division for the common platform generation. Then the adaptability measure is used to find the optimal case in the database. The measure has been used in the gasoline saw common platform design process.