A method, a device, and a system for backup are disclosed. The method includes: receiving a backup packet; analyzing the backup packet to obtain a serial number of an active data packet, where the serial number is carried in the backup packet; calculating a sum of the serial number of the active data packet and a specific increment value to obtain a serial number of a standby data packet; and backing up the serial number of the standby data packet. Therefore, after the service is switched over from the active device to the standby device, the active device still regards the data packet sent by the former standby device as a new data packet and receives the data packet, which prevents service interruption caused by the switchover and improves continuity of service processing in the process of two-node cluster hot backup.