Aiming at rapid response to requirements of mechanical transmission products customization, a system for rapid locking assembly variant design was proposed in terms of features of locking assembly. The bases of the system are Product Configuration Model (PCM) and Case-Based Reasoning (CBR). According to the system, PCM of locking assembly was built by extending traditional Generic Bill of Materials (GBOM). In addition, integration of PCM and CBR was completed by combining method of transforming PCM to a case with similarity assessment algorithm. Moreover, a rapid variant design software system based on PCM and CBR for locking assembly was developed. Finally, the software system was applied to design new locking assembly. Results demonstrate that it is effective for PCM-CBR to support rapid response to customization.