This paper presents an optimistic protocol based on the scheme of public verifiable secret share for distributed fair exchange. A distributed third party is designed to guarantee the fairness of exchange. Comparing to traditional trusted third party, this distributed third party is not fixed but dynamically selected by the players of exchange. This solution can effectively remove the risk of single point of failure and defend compromising attacks. The results of protocol analysis show that the protocol guarantees strong fairness and effectiveness of up to one-half malicious participants in distributed third party.