A method for processing registration backup data, comprising: receiving, by a Serving Call Session Control Function, S-CSCF, a registration request message that carries at least a user identity and deregistration information from a User Equipment, UE; and determining, by the S-CSCF, that the S-CSCF itself stores no registration information corresponding to the user identity, and determining, by the S-CSCF, whether the registration request message carries a "reg-id" parameter, wherein the "reg-id" parameter is used to indicate that the UE supports multi-registration; if said "reg-id" parameter is carried in the registration request message, then sending, by the S-CSCF, a first Server Assignment Request, SAR, message to a Home Subscriber Server, HSS, wherein the first SAR message carries the user identity and a first indication information, wherein the first indication information includes first server assignment type information which requests registration backup data corresponding to the user identity from the HSS; receiving, by the S-CSCF, from the HSS an Server Assignment Answer, SAA, message which carries registration backup data corresponding to the user identity; comparing, by the S-CSCF, whether the registration backup data returned by the HSS is the same as registration information in the registration request message; if the registration backup data returned by the HSS is different from the registration information in the registration request message, using, by the S-CSCF, the registration information in the registration request message to update the registration backup data returned by the HSS, and sending the updated registration backup data to the HSS for backup through a second SAR message; if the registration backup data returned by the HSS is the same as the registration information in the registration request message, sending, by the S-CSCF, a third SAR message to the HSS, wherein the third SAR message carries the user identity and a second indication information, wherein the second indication information includes second server assignment type information which instructs the HSS to deregister the user identity; if said "reg-id" parameter is not carried in the registration request message, then sending a fourth SAR message to the HSS, wherein the fourth SAR message carries the user identity and a third indication information, and the third indication information comprises third server assignment type information which instructs the HSS to deregister the user identity. receiving, by a Serving Call Session Control Function, S-CSCF, a registration request message that carries at least a user identity and deregistration information from a User Equipment, UE; and determining, by the S-CSCF, that the S-CSCF itself stores no registration information corresponding to the user identity, and determining, by the S-CSCF, whether the registration request message carries a "reg-id" parameter, wherein the "reg-id" parameter is used to indicate that the UE supports multi-registration; if said "reg-id" parameter is carried in the registration request message, then sending, by the S-CSCF, a first Server Assignment Request, SAR, message to a Home Subscriber Server, HSS, wherein the first SAR message carries the user identity and a first indication information, wherein the first indication information includes first server assignment type information which requests registration backup data corresponding to the user identity from the HSS; receiving, by the S-CSCF, from the HSS an Server Assignment Answer, SAA, message which carries registration backup data corresponding to the user identity; comparing, by the S-CSCF, whether the registration backup data returned by the HSS is the same as registration information in the registration request message; if the registration backup data returned by the HSS is different from the registration information in the registration request message, using, by the S-CSCF, the registration information in the registration request message to update the registration backup data returned by the HSS, and sending the updated registration backup data to the HSS for backup through a second SAR message; if the registration backup data returned by the HSS is the same as the registration information in the registration request message, sending, by the S-CSCF, a third SAR message to the HSS, wherein the third SAR message carries the user identity and a second indication information, wherein the second indication information includes second server assignment type information which instructs the HSS to deregister the user identity; if said "reg-id" parameter is not carried in the registration request message, then sending a fourth SAR message to the HSS, wherein the fourth SAR message carries the user identity and a third indication information, and the third indication information comprises third server assignment type information which instructs the HSS to deregister the user identity.