In Peer-to-Peer(P2P) streaming system, the multicasting tree construction method influences considerably on network load. Under current available strategies, network resources are not used economically, or network resource friendship and the flexibility can't be approached thoroughly. With the increasing application of Internet, network infrastructure itself becomes a precious resource, which should be performed effectively. In our implementation, network transmission delay between the peers is detected, used as practical performance metrics of the P2P streaming system. According to the metrics, network friendly tree is provided as overlay multicasting tree construction strategy for P2P streaming system. In this strategy, additional transmission delays are minimized as new peers enter. The simulation experiment presents that the proposed strategy network friendly tree works better than other contrasts. And the transmission performance improves significantly at minor additional cost.