A method for synchronizing user protocol messages between service nodes, comprising: when receiving a user-related protocol message, a service node judging whether a state of a backup group to which a user belongs corresponding to the received user-related protocol message is active or standby on the service node, and according to a type of the user-related protocol message, determining whether to send the user-related protocol message to another service node on which the state of the backup group is standby or active; and when determining to send the user-related protocol message, the service node encapsulating the user-related protocol message through a service backup protocol and sending the encapsulated user-related protocol message to the other service node; characterized in that when the type of the user-related protocol message received by the service node is a protocol message from an Authentication Authorization and Accounting, AAA, server, a Dynamic Host Configuration Protocol, DHCP, server or a portal authentication protocol, Portal, server, if the state of the backup group to which the user belongs corresponding to the user-related protocol message is standby on the service node, the service node sending, according to said determining and encapsulating, the user-related protocol message to the other service node on which the state of the backup group is active; or when the type of the user-related protocol message received by the service node is a Layer 2 Tunneling Protocol, L2TP, tunnel control message, if the state of the backup group to which the user belongs corresponding to the L2TP tunnel control message is active on the service node, the service node sending, according to said determining and encapsulating, the L2TP tunnel control message to the other service node on which the state of the backup group is standby. when receiving a user-related protocol message, a service node judging whether a state of a backup group to which a user belongs corresponding to the received user-related protocol message is active or standby on the service node, and according to a type of the user-related protocol message, determining whether to send the user-related protocol message to another service node on which the state of the backup group is standby or active; and when determining to send the user-related protocol message, the service node encapsulating the user-related protocol message through a service backup protocol and sending the encapsulated user-related protocol message to the other service node; characterized in that when the type of the user-related protocol message received by the service node is a protocol message from an Authentication Authorization and Accounting, AAA, server, a Dynamic Host Configuration Protocol, DHCP, server or a portal authentication protocol, Portal, server, if the state of the backup group to which the user belongs corresponding to the user-related protocol message is standby on the service node, the service node sending, according to said determining and encapsulating, the user-related protocol message to the other service node on which the state of the backup group is active; or when the type of the user-related protocol message received by the service node is a Layer 2 Tunneling Protocol, L2TP, tunnel control message, if the state of the backup group to which the user belongs corresponding to the L2TP tunnel control message is active on the service node, the service node sending, according to said determining and encapsulating, the L2TP tunnel control message to the other service node on which the state of the backup group is standby.