Most traditional trust computing models in E-commerce do not take the transaction frequency among participating entities into consideration, which makes it easy for one party of the transaction to obtain a high trust value in a short time, and brings many disadvantages, uncertainties and even attacks. To solve this problem, a transaction frequency based trust is proposed in this study. The proposed method is composed of two parts. The first part is built on the classic Bayes analysis based trust models which are ease of computing for the E-commerce system. The second part is the transaction frequency module which can mitigate the potential insecurity caused by one participating entity gaining trust in a short time. Simulations show that the proposed method can effectively mitigate the self-promoting attacks so as to maintain the function of E-commerce system.