Abstract: In light of the tunnel construction safety and gas fortification, the regional geological background and gas structure of the Longquan Mountain in Sichuan Basin were investigated from a macro perspective, the Longquanshan tunnel of Chengdu Metro was identified as a gas tunnel in non-coal measure strata considering the development and distribution of geological structure at tunnel site, and its macroscopic characteristics of gas structure were presented. Based on the detailed exploration results of gas detection, geophysical exploration and laboratory test of boreholes, the microscopic characteristics of gas buried depth, elevation, strata and rock mass of the Longquanshan tunnel were stated. By comparing with the Longquanshan water tunnel to be completed soon, the above mentioned characteristics were proved by gas monitoring results, and a prediction and evaluation of gas occurrence and fortification grade of the Longquanshan tunnel were conducted.