A method for accessing a mobile network, comprising: receiving a user identity that is for a UE to access the mobile network, wherein the user identity for the UE to access the mobile network comprises an MSISDN and an IMSI, and is associated with a user identity for the UE to access a WLAN, wherein the user identity for the UE to access the WLAN comprises a user name and a password and the user name of the UE is the MSISDN; and enabling the UE to access the mobile network according to the user identity for the UE to access the mobile network, in that the receiving a user identity that is for a UE to access the mobile network and the enabling the UE to access the mobile network according to the user identity for the UE to access the mobile network are executed by a Trusted WLAN Access Gateway, TWAG, wherein the user name and the password for the UE to access the WLAN are obtained by an AAA server or an AAA proxy, and the user name of the UE is the MSISDN, and the IMSI associated with the MSISDN are obtained by the AAA server or the AAA proxy from an HLR or an HSS according to the MSISDN, and the IMSI and the MSISDN are sent by the AAA server or the AAA proxy to the TWAG through an accounting response message to enable the UE to access the mobile network and the enabling the UE to access the mobile network according to the user identity for the UE to access the mobile network comprises: obtaining a local IP for the UE to access the WLAN; determining, according to an APN, a GGSN for accessing the mobile network, and sending a request for establishing a GTP tunnel to the GGSN, wherein the request carries the user identity for the UE to access the mobile network, and the user identity for the UE to access the mobile network comprises the MSISDN and the IMSI; obtaining a remote IP that is for the UE to access the mobile network and is assigned by the GGSN; and establishing a correspondence between the local IP and the remote IP, so as to enable the UE to access the mobile network, wherein the method further comprises: receiving a service packet, and performing network address translation NAT between the local IP and the remote IP on the service packet. receiving a user identity that is for a UE to access the mobile network, wherein the user identity for the UE to access the mobile network comprises an MSISDN and an IMSI, and is associated with a user identity for the UE to access a WLAN, wherein the user identity for the UE to access the WLAN comprises a user name and a password and the user name of the UE is the MSISDN; and enabling the UE to access the mobile network according to the user identity for the UE to access the mobile network, in that the receiving a user identity that is for a UE to access the mobile network and the enabling the UE to access the mobile network according to the user identity for the UE to access the mobile network are executed by a Trusted WLAN Access Gateway, TWAG, wherein the user name and the password for the UE to access the WLAN are obtained by an AAA server or an AAA proxy, and the user name of the UE is the MSISDN, and the IMSI associated with the MSISDN are obtained by the AAA server or the AAA proxy from an HLR or an HSS according to the MSISDN, and the IMSI and the MSISDN are sent by the AAA server or the AAA proxy to the TWAG through an accounting response message to enable the UE to access the mobile network and the enabling the UE to access the mobile network according to the user identity for the UE to access the mobile network comprises: obtaining a local IP for the UE to access the WLAN; determining, according to an APN, a GGSN for accessing the mobile network, and sending a request for establishing a GTP tunnel to the GGSN, wherein the request carries the user identity for the UE to access the mobile network, and the user identity for the UE to access the mobile network comprises the MSISDN and the IMSI; obtaining a remote IP that is for the UE to access the mobile network and is assigned by the GGSN; and establishing a correspondence between the local IP and the remote IP, so as to enable the UE to access the mobile network, wherein the method further comprises: receiving a service packet, and performing network address translation NAT between the local IP and the remote IP on the service packet. obtaining a local IP for the UE to access the WLAN; determining, according to an APN, a GGSN for accessing the mobile network, and sending a request for establishing a GTP tunnel to the GGSN, wherein the request carries the user identity for the UE to access the mobile network, and the user identity for the UE to access the mobile network comprises the MSISDN and the IMSI; obtaining a remote IP that is for the UE to access the mobile network and is assigned by the GGSN; and establishing a correspondence between the local IP and the remote IP, so as to enable the UE to access the mobile network, wherein the method further comprises: receiving a service packet, and performing network address translation NAT between the local IP and the remote IP on the service packet.