User data migration method and user data backup method, device and system

The embodiments of the invention disclose a user data migration method and a user data backup method, device and system, and belongs to the technical field of broadband access. The user data migration method is applied to a target BRAS (Broadband Remote Access Server) included in a broadband remote access system, and comprises the steps of: when an MPU (Microprocessor Unit) receives a migration message sent when a first target VM (Virtual Machine) satisfies a configured preset migration condition, selecting a VM satisfying a first preset condition from all online VMs of multiple BRASs, and determining the selected VM as a second target VM; the MPU generating a migration rule based on VLAN (Virtual Local Area Network) information of the first target VM and VLAN information of the second target VM, and sending the migration rule to a switch, so that when the switch receives a message sent to the first target VM, the switch forwards the message to the second target VM based on the migration rule, i.e., user data