The selfishness of nodes degrades the system usability of P2P network. Debt relationship based file exchange network constructs an incentive mechanism which induces cooperation and guarantees fairness in file exchange. The key point of the mechanism is finite neighbors, an inherent characteristic in DHT (distributed hash table) networks and so is the interacting between nodes form a repeated games. DFFE (debt relationship based fair file exchange in DHT network) protocol only needs to maintain a little local interacting information, so the protocol cost is low and scalable for large network. In routing, one-hop information based greedy arithmetic is used. Game among rational nodes exists a Nash equilibrium and the approximate algorithm of strategy selection gradually converges. Simulations indicate the validity of incentive mechanism and the steady performance in dynamic networks.