In the e-commerce transaction process, varieties of network fraud problems occurred frequently. A C2C service reputation evaluation model under Cloud computing is Proposed. Firstly, the paper analyzes the four factors impact reputation evaluation under the cloud computing environment, and analysis the weights of each factor quantitatively. Secondly, a method to calculate the long-time reputation, short-time reputation and the accumulated reputation is proposed in this paper. Finally, examples show that the C2C service reputation evaluation model cloud computing environment can effectively reduce the number of online fraud and improve the security of online transactions.