Abstract: In view of the complicated air leakage and the low oxygen concentration and the high harmful gas concentration in the upper corner under the U+L ventilation condition in Suancigou Coal Mine, the air leakage law and the air concentration distribution in the goaf are studied through the field test and numerical simulation, and a new sealing measure is proposed. The results show that the air leakage in the contact lane of goaf causes the harmful gas in the adjacent goaf to pass through the sealed fracture to leak into the goaf of the stope; under the influence of residual coal oxidation and raw coal CO₂ desorption on the stoping face, O₂ concentration in the return corner is low, CO₂ and nitrogen concentration is high, O₂ concentration in the return corner is directly proportional to the distance between the closed contact lane and the working face, while CO₂ and N₂ concentration are inversely proportional to the distance between the closed contact lane and the working face; after the adoption of closed sealing measures, the CO₂ concentration in the upper corner is reduced to less than 0.25%, and the O₂ concentration is increased to more than 16%. The range of goaf “three zones” obtained by field test and numerical simulation is basically the same, and the oxidation zone ranges from 7 m to 110 m. It is determined that the coal cutting speed of the sheaf shall not be less than 2.13 m/d.