Abstract: The mining of inclined coal seams is often accompanied by the problem of conveyor upward and downward movement. Generally, the pseudo-slope lengths (PSLs) were increased to ensure the normal movement of the hydraulic support and to provide more working space for the headstock. The increase of PSLs would affect the change of gas in the inter-frame and upper corner, resulting in abnormal gas concentration at the working face. To address the problem of gas exceedance in the upper corner of inclined thick coal seam mining, Fluent numerical simulation software was applied to study the gas distribution and flow field in the U-shaped ventilation method from the PSLs of 20 m, 25 m, 30 m, and 35 m respectively. The simulation found that the PSL, to a certain extent, affects the wind flow in the mining area, making the wind flow in the mining area disorderly, and the gas collects seriously in local areas, which in turn has a certain influence on the gas distribution and transportation in the mining area. The research results showed that: the PSLs had a significant influence on the gas concentration distribution in the mining area, with the increase of PSLs, the wind flow in the mining area would turn in advance, resulting in the gas in the mining area gushing out to the working face in advance, and the gas abnormal area will gradually shift from the upper corner to the working face, and the gas concentration in the upper corner will gradually decrease. After optimizing the application of the simulation results to the PSLs in the field, the maximum gas concentration in the upper corner and backwind tunnel was controlled within 1% at the PSLs of about 25m in the test working face, and there was no abnormal gas accumulation in the working face. The simulation results match the field observation, and the optimal PSL is 25 m.