To solve the problem of the transaction security in C2C e-commerce, this paper proposes an evaluation model based on cloud model theory. The created definition of the trust cloud and the standard cloud in the valuation model show the ambiguity and random of the trust and realize transferring quantitative data to qualitative data. This paper puts forward the synthesis, similarity operation and realizes comprehensive evaluation and decision-making based on trust classification. The results of experiments show that the trust evaluation model proposed is valid.