Abstract: In order to achieve a secure network transactions, the main problem we are facing to is the trust, risk and other issues. In this paper, taking the network transactions of complex network environment as the researching background, we proposed the trust evaluation model based on cloud model. This trust evaluation model can implement the conversion between qualitative and quantitative of trusts, and it can objectively reflect the randomness, fuzziness, and unpredictability of the trusts. In order to effectively evaluate transaction risk in complex networks, the risk evaluation approach based on cloud model is researched and proposed. The simulation results confirm that this trust evaluation model can make a reasonable evaluation of the entities' trust in the complex network environment, and that the risk evaluation approach researched in this paper can reasonably predict the risk of network transactions. Finally, we design and realize a risk evaluation system based on cloud model, further also confirm the feasibility and rationality of the proposed risk evaluation approach.