<p>Mountain tunnels have the characteristics of long construction period, multiple uncontrollable factors, and high construction difficulty. To investigate collapse risks associated with complex geological conditions, a deep-buried mountain tunnel in Baise, Guangxi, China, was selected as a case study. The risk of tunnel collapse during construction was analyzed using theoretical analysis, numerical simulation, and field monitoring. The research results indicate that: (1) using intuitionistic fuzzy theory and skewed normal weighing function can better evaluate the risk of mountain tunnel collapse. The analysis shows that the consequence level of Baise Tunnel is III (severe); (2) The construction of three-bench method with temporary cross braces, combined with long anchor rod support and deep grouting, can effectively control the deformation of surrounding rock; (3) Based on the risk assessment and numerical simulation results, several on-site construction measures were proposed to mitigate the risk of mountain tunnel collapse. These include segmented excavation at tunnel entrances, increased monitoring frequency, and timely closure of the surrounding rock. The research results can effectively improve the safety of tunnel construction, save energy and costs, and promote the transformation of engineering construction towards green and low-carbon.</p>