The invention provides a method for supporting WLAN roaming switching between different subnets, a device and a system; a communication network system comprises: a mobile site, an access control device which corresponds to a plurality of subnets and a wireless access point which is connected with the access control device, wherein, the mobile site sends a switching request to the local subnet through the roaming switching, the access control device can obtain addressable information of the access control device which corresponds to a target wireless access point in a non-local subnet according to the switching request; then, routing forwarding of the switching request is conducted between the local subnet and the access control device of the non-local subnet; meanwhile, the invention also provides a server which registers, manages, and inquires the access control device that corresponds to the plurality of subnets and the information of the wireless access point which is connected with the access control device.