Embodiments of the present invention provide a method, an apparatus, and a network system for a terminal to traverse a private network to communicate with a server in an IMS core network. The method includes: the terminal sets a source address of service data to be sent as a virtual IP address, sets a destination address of the service data to be sent as an address of an internal network server, and obtains a first service packet, where the virtual IP address is an address allocated by the IMS core network to the terminal, encapsulate the first service packet into a first tunnel packet, and send the first tunnel packet to the security tunnel gateway over a VPN tunnel between the terminal and a security tunnel gateway, then the security tunnel gateway sends the first service packet in the first tunnel packet to the internal network server.