Online trading reputation evaluation mechanism achieve the fairness of results based on the realization of the fairness of rights, the fairness of reputation evaluation process and the fairness of reputation limit. After the present reputation evaluation mechanism have resolve d the fairness of rights and the fairness of process, this study determine the relative fair reputation limit by the method of Logarithm of transaction amounts through the design of Weber-Fechner law. The results show that the application of new method not only can achieve the relative fairness in the same monetary environment of transaction, but also can easily realize the conversion of reputation limit between the different trading monetary environments. The new method can be flexibly used in the application of online trade reflecting the fairness of transaction to both parties in the online trade.