The invention discloses a method for building an NAT crossing channel by a system terminal unit, which comprises the following steps: the terminal unit of a calling party system respectively obtains a data receiving port and an network address on forwarding service nodes, the data receiving port and the network address on an NAT unit, and the data receiving port and the network address on the system terminal unit by the forwarding service nodes and sends the data receiving ports and the network addresses to the terminal unit of a called party system; a data sending port of the terminal unit of the called party system respectively carries out a through testing, a forwarding testing and a crossing testing in sequence towards the terminal unit of the calling party system and obtains the network address and the port of the data receiving port of the terminal unit of the calling party system; the network address and the port received by the data receiving port of the terminal unit of the called party system are sent to the terminal unit of the calling party system. The method for building the NAT crossing channel by the system terminal unit of the invention takes the communication diversity of multilevel NAT from terminal to terminal into consideration, makes the most of network resources in a private network formed by multilevel NAT and can realize the crossing of NAT under a plurality of occasions.