Abstract: To solve the problem of gas over limit in return airflow caused by a large amount of gas emission in the goaf of a fully mechanized mining face with large dip angle downward ventilation, the unit measurement method was used to divide the fully mechanized mining face into several units from the intake air to the return air, and the air leakage, gas emission, and gas concentration between each unit and the goaf were obtained. The distribution characteristics of the abnormal gas accumulation area in the goaf were analyzed and evaluated. Using the numerical simulation software, the gas extraction effect of high-level borehole under different layout conditions was analyzed, and the extraction parameters of high-level borehole were optimized. The research results show that under the influence of the total air pressure difference at both ends of the goaf, the negative pressure of high-level borehole extraction, the hot air pressure in the goaf, and the gas buoyancy lift force, the goaf with a flat distance of 10 m to 30 m near the inlet side and a distance of 10 m to 25 m from the coal seam roof forms an abnormal gas accumulation area. When the final hole of the high-level borehole is arranged in the abnormal gas accumulation area and the extraction flow rate is 60 m³/min, the gas extraction effect is optimal, and the hidden danger of gas over limit in return air flow is eliminated.