A method, an apparatus, and a system for processing registration backup data are disclosed. A Serving Call Session Control Function (S-CSCF) receives a request from a User Equipment (UE) and judges whether the request carries multi-registration information so as to decide whether to add multi-registration indication information into a Server Assignment Request (SAR) message sent to a Home Subscriber Server (HSS). The HSS performs operations according to whether the SAR message carries the multi-registration indication information, therefore avoiding unnecessary signaling interactions, reducing the signaling load, avoiding unnecessary data processing between the HSS and the S-CSCF, and saving system resources.