As Peer-to-peer(P2P) file sharing is increasingly becoming popular as a new paradigm for file exchange,its security has been an issue of concern,and access control is one of the key technologies of system security.However,the decentralized and dynamic characteristic of P2P system makes the traditional access control methods cannot be applied directly,and the existing access control methods for P2P file-sharing system cannot be a good solution to against the large concurrent file-downloading request or bombarded malicious file-downloading requests.To address the above problems,a kind of access control mechanism based on commodity market(ACMCM) in P2P file sharing system is presented in this paper.The market mechanism makes the file-downloading requests in a balanced distribution among the file providers;the price of downloaded file increases in exponential with the number of repeat downloading on same file,so the malicious file downloading is effectively denied;the bandwidth is allocated rationally among the concurrent downloading,and it makes system time used for file-downloading be short as possible.We believe that ACMCM preserves P2P decentralized structure and dynamic property.Finally,the distributed protocol of the major processes is presented.