A User Equipment (UE), network-side device, system and method are disclosed for external authentication using an Extensible Authentication Protocol (EAP). The method includes, when the UE is initially attached to an Evolved Packet System (EPS) network via an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), the UE transmitting EAP authentication information required for the EAP authentication to a packet data network gateway and, after receiving EAP request, the UE transmitting a bearing resource modification request message carrying EAP response to the packet data network gateway. With the method, the UE can use the EAP authentication to implement authentication and authorization by an external authentication and authorization server via a GGSN/PDN GW in a process of connecting the UE to the EPS via a 3GPP access.