Mobile network access method, UE, security service gateway, and system

The invention discloses a mobile network access method comprising that user equipment (UE) acquires information relative to the UE itself from a core network and selects the type of an accessed mobile network according to the information. The invention also discloses the UE, a security service gateway, and a system. The method, the UE, and the system may satisfy the requirements of different terminals and different application services in order that it is easier and higher efficient for the UE to obtain the service of the mobile network.