Abstract: The optimistic e-commerce protocol is made up of multiple sub-protocols，and is significantly different from traditional authentication protocol.The main purpose of the e-commerce protocol is to realize fair exchange between both buyers and sellers who are assumed to be dishonest.That is，it is necessary for those buyers and sellers to prevent the internal attack.Based on the deep analysis of each attribute in fair exchange protocols，the authentication attribute of item is defined.The fairness that should be met by e-commerce protocols is defined，and an extended SVO logic syntax which takes an optimistic e-commerce protocol as an object is proposed to demonstrate the fairness analysis method.The security vulnerabilities and improvement suggestions of the e-commerce protocol are also discussed.