The availability of a P2P (peer-to-peer) file sharing system is heavily affected by the high churn rate of users. When the largest P2P file sharing system Maze in CERNET is developed, the log collected in the system is used to get a better understanding of the users’ characteristics, to find the key factor which influences the resource availability, and to instruct the future development of Maze system. In this paper, the concept of P2P file sharing systems’ availability is redefined from users’ perspective. With the log of Maze, it is the first study to use clustering technique to quantitatively categorize users in a P2P file sharing system. Based on the thorough study of behavior of the active users amounting to 0.77% of the total users and the impact on Maze’s availability, this paper concludes that this kind of users plays a similar role to server so that they greatly increase the availability of system and are feasible resource to improve the performance of P2P file sharing systems.