Abstract: Low oxygen and excessive harmful gas in upper corner are easy to occur in the lower mining face under sealed fire area of small coal mine, which seriously restricts the safe production of coal mine. To clarify the migration law of harmful gas in goaf, a mathematical model of gas migration in goaf was derived based on Darcy’s law, diffusion law and hydrodynamic dispersion law. The migration process of harmful gas in goaf in 13102 working face of Shaping Mine was simulated using COMSOL Multiphysics. The results show that the fresh air in the inlet roadway and working face leaks into the goaf, and then flows out from the upper corner of the return air roadway. Driven by the fresh air, CH₄ and CO flows out from the upper corner, resulting in excessive CH₄ and CO and low oxygen at the upper corner of the working face. On this basis, the measures of buried pipe for extraction at upper corner and balanced pressure ventilation are put forward to prevent low oxygen and harmful gas at the corner of the working face. The buried pipe for extraction at upper corner can only prevent the problem of low oxygen in the working face, while the balanced pressure ventilation can prevent the low oxygen in the working face and prevent the harmful gas from flowing to the working face.