Abstract: This paper studies the size of narrow pillar inside the section coal pillar via a method that combines the theory calculation of inner stress field, field test and FLAC^3D numerical simulation by taking the layout of 010203 working face gas drainage roadway of Baijigou coal mine as the research object in order to determine the size of coal pillar reasonably while arranging the gas drainage roadway inside the section coal pillar along the goaf.According to the research results, it can be known from the field test and theory analysis that the distance from the distribution range of internal stress field to the goaf is about 0-7 m, within which, the stress of coal is relatively low.The distribution feature of the stress inside the section coal pillar under the narrow coal pillar of different sizes can be known based on the numerical simulation results.The vertical stress in the narrow coal pillar increases and gas roadway is influenced more by the working face mining along with the increase of narrow coal pillar width.The inner failure characteristics of coal pillars of different sizes can be obtained based on the distortion rule of roadways in different driving stages.When the coal pillar is 5 m, the deformation of roadway is far less than other circumstances after the recovery of each layer, though the narrow pillar side may be deformed substantially during the roadway driving period.Finally, the reasonable size of the narrow pillar is 5 m, which is calculated by considering the realities of this mine, which provides some reference for determining the size of narrow pillar under similar geological conditions.