As the key part of a water diversion project, the diversion tunnel determines the quality of the whole water diversion project. In order to avoid unnecessary economic losses during operation, a safety evaluation index system for diversion tunnels was established based on the cold and arid climate in Northwest China, and the unascertained measure theory (UMT) model was used to evaluate the structural safety. The improved G2-anti-entropy weight (AEW) method was used to calculate the subjective and objective weights. The Lagrangian optimal multiplier method was devoted to perform the subjective and objective combination weighting. Based on the unascertained measure theory, the confidence identification criterion was performed to identify the security level. The model was applied to evaluate the safety of four typical tunnel sections in Datonghe to Qinwangchuan Water Diversion Project, and the safety level of each section was obtained. The evaluation results showed that the structures of sections 4+757.50~4+797.00 and 5+071.00~5+135.00 are slightly damaged, and that of sections 4+247.01~4+281.60 and 6+240.00~6+346.26 are seriously damaged. Through the argumentation, the results are consistent with the actual project situation, which indicates that the model is effective and practical for the safety assessment in the northwest region, and can be applied to the safety evaluation of long-distance water transport projects.