A case study is conducted on the Zhaxi tunnel to ensure the safe excavation of gas outburst tunnels and investigate the gas outburst elimination mechanism by drainage drilling holes. Theoretical analysis and COMSOL simulation methods are comprehensively employed to examine the variation rules of pressure, content, and effective drainage radius of gas in coal seam around the drainage holes. The obtained results are compared with the monitoring data of gas drainage on site, as follows. (1) The gas pressure drastic reduction zone, significant reduction zone, slow reduction zone, and original pressure zone are formed in the coal seam body around the drainage holes, which have an effective drainage radius of 2.58 m. (2) The gas content and pressure in the coal seam body gradually decrease with the increase in distance to the drainage holes, but the effective drainage radius expands, which is exponentially related to the cumulative drainage time. (3) The gas drainage efficiency increases by ~0.5% when the angle between a single row drilling holes and the coal seam increases by 5°. 〖ＪＰ２〗The maximum residual gas content in the coal seam is found to be 5.95 m3/t after gas drainage of 70 advance gas drainage holes along the bed, indicating a significant reduction effect of gas.