Aiming at the problem of uncertain factors in network trust evaluation, under the research background of security trading in complex open network, the extension cloud theory was introduced. Using the matter-element theory of extenics and the uncertainty of cloud model and the advantage of both the quantitative and qualitative analysis, an extension cloud-based network trust evaluation model was proposed. In the proposed model, the transformation between qualitative trust value and quantitative trust value was realized. And then, an evaluation method based on the extension cloud was put forward on the basis of the proposed model. The trust assessment of network security trading can be achieved effectively for providing good basis for final trust decision. The simulation experimental results show that, the trust evaluating and scheduling algorithm has improved the accuracy of trust evaluation and the successful rate of transaction in complex network environment and alleviated the network transaction entity fraud effectively. The proposed network trust extension method is effective and feasible.