<p>Due to the complexity of the occurrence mechanisms of high-pressure gas reservoirs in magmatic rock strata, there is still no unified understanding of gas migration and release characteristics in the surrounding rock during tunnel crossings, which hampers effective guidance for tunnel gas release measures. Field monitoring of CO₂ release in a tunnel on the Qinghai–Tibet Plateau revealed mechanisms of hazardous gas generation and storage in magmatic rock. Based on the monitoring results, the gas release characteristics at the tunnel face were classified into three stages: stable release stage of the reservoir, rapid decline stage of the reservoir, and fracture-dominated stage. A numerical approach was developed to model gas migration in surrounding rock using the ideal gas law while considering reservoir characteristics. Results indicate that gas release from magmatic rock reservoirs is abrupt, with flow rates in fractures initially increasing rapidly and then decreasing slowly after excavation. When the tunnel face does not intersect the reservoir fractures, gas release primarily comes from the rock. However, once the tunnel face intersects the fractures, over 99.9% of gas released originates from reservoir fractures. The efficiency of gas release is significantly improved by exhaust holes only when the tunnel face intersects the reservoir fractures. The effectiveness of these holes is positively related to the effective exhaust area and fracture density. In addition, the installation of exhaust holes may inhibit fracture flow velocity, though this effect diminishes over time.</p>