Abstract: In order to determine the reasonable size of segment coal pillar, the theoretical calculation formula of plastic failure zone on both sides of the coal pillar was established on the premise that the width of the coal pillar need to maintain its stability.Three schemes were proposed that the width of coal pillar was respectively 27 m, 30 m and 33 m combined with field measurement data.The stress field and plastic deformation characteristics of coal pillar and roadway during the mining of working face in the lower section were analyzed by FLAC^3D numerical software. The research results showed that when the segment coal pillar is 27 m wide, the stress concentration is obvious on both sides of the segment pillar, the plastic failure envelopes bolt deeply and the edge of the roadway are in the stress area, which is not conducive to the stability of roadway; the surrounding rock condition of the roadway is obviously improved when the width of coal pillar reaches 30 m or 33 m. Considering the principles of three "favorable", the reasonable width of segment coal pillar in 2207 working face of Changcun Coal Mine is determined to be 30 m.