The employment relates to a realization method for establishing peer-to-peer communication connection, a method, an apparatus and a system for realizing network communication by traversing NAT. The employment mainly comprises following steps. When the user terminal may not directly communicate with the peer user terminal by the manner of peer-to-peer communication for needing to traverse through the NAT (network address translator) entity, the type information of the NAT entity needed to be traversed is determined. Then, the communication connection with the peer user terminal is established according to the determined type information of the NAT entity and by using the predefined manner of peer-to peer communication and the communications between the user terminals are carried out. The employment realizes that the manner of peer-to-peer communication is adopted as much as possible during the process of traversing the NAT to carry out the communication so as to reduce the occupation of server source and effectively save the costs of network operation. The employment also tries to use the corresponding re-issue mechanism during the procedure of establishing the peer-to-peer communication in order to resolve various connection establishment problems when the factors which may affect the connection establishment appear.