Abstract: To solve the problem that BitTorrent networks produce a large volume of traffic and put great pressure to network operator, a BitTorrent working mechanism is researched, and a content cache service system for BitTorrent networks is designed. The system includes a traffic monitoring and administrating system and a content cache server, which can automatically download the popular files and upload it to the peers in a given range of network. Experiment results show that the system can reduce the traffic across the border of networks remarkably.