An access method, comprising: receiving, by a network side device of a first network, registration information sent by a plurality of second network access points of a second network, wherein the registration information sent by each second network access point includes an identifier of a second network access point and further includes position information of the second network access point and an IP address of the second network access point; establishing, by the network side device, a mapping relationship between the identifiers of the plurality of the second network access points and the first network; obtaining, by the network side device, an identifier of at least one second network access point, wherein a second network access point corresponding to the identifier of the at least one second network access point can perform air interface offloading on a first network in which a mobile device is currently located and was registered with the network side device in advance; generating, by the network side device, according to the obtained identifier of the at least one second network access point, a first message that carries the identifier of the at least one second network access point; and sending, by the network side device, the first message to the mobile device, wherein when the first message carries identifiers of a plurality of second network access points the generating, by the network side device, according to the obtained identifier of the at least one second network access point, a first message comprises: determining, by the network side device, the first network currently accessed by the mobile device; obtaining, by the network side device, identifiers of the plurality of second network access points according to position information of the mobile device, position information of each second network access point and load information of each second network access point from identifiers of second network access points corresponding to the first network, and generating a first message that carries the identifiers of the plurality of second network access points. receiving, by a network side device of a first network, registration information sent by a plurality of second network access points of a second network, wherein the registration information sent by each second network access point includes an identifier of a second network access point and further includes position information of the second network access point and an IP address of the second network access point; establishing, by the network side device, a mapping relationship between the identifiers of the plurality of the second network access points and the first network; obtaining, by the network side device, an identifier of at least one second network access point, wherein a second network access point corresponding to the identifier of the at least one second network access point can perform air interface offloading on a first network in which a mobile device is currently located and was registered with the network side device in advance; generating, by the network side device, according to the obtained identifier of the at least one second network access point, a first message that carries the identifier of the at least one second network access point; and sending, by the network side device, the first message to the mobile device, wherein when the first message carries identifiers of a plurality of second network access points the generating, by the network side device, according to the obtained identifier of the at least one second network access point, a first message comprises: determining, by the network side device, the first network currently accessed by the mobile device; obtaining, by the network side device, identifiers of the plurality of second network access points according to position information of the mobile device, position information of each second network access point and load information of each second network access point from identifiers of second network access points corresponding to the first network, and generating a first message that carries the identifiers of the plurality of second network access points. determining, by the network side device, the first network currently accessed by the mobile device; obtaining, by the network side device, identifiers of the plurality of second network access points according to position information of the mobile device, position information of each second network access point and load information of each second network access point from identifiers of second network access points corresponding to the first network, and generating a first message that carries the identifiers of the plurality of second network access points.