Abstract: In the construction of railway tunnel, many need to pass through the gas stratum so that the safety risk of construction is quite high. A railway tunnel gas risk assessment model based on the unascertained measurement theory was established to improve the accuracy of tunnel gas risk assessment. Yudu No.2 tunnel project of Xingquan railway was taken as a case study. Nine factors affecting gas risk were selected as evaluation indexes, and their grading standards were established. The weight calculated by AHP and entropy weight method was coupled into comprehensive weight by using the method of multiplication normalization, and then the comprehensive weight and the unascertained measure matrix of evaluation index were used to determine the multi index comprehensive measure evaluation vector. Finally the risk level of tunnel gas was determined according to the confidence recognition criteria. The results showed that: compared with the traditional evaluation model, this evaluation model could improve the accuracy of tunnel gas risk evaluation, and it was highly consistent with the actual construction situation, and could be used as an effective evaluation model for gas risk in similar tunnel projects.