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 migration of the first target VM is realized, and then a physical server corresponding to the first target VM can be powered off. Therefore, the power consumption of the system is reduced, and idle system resources are simultaneously avoided.