The width of section coal pillar is an important control factor affecting the stability of gob-side entry and the development of fracture structure of overlying strata. In order to clarify the stress and deformation characteristics of coal pillars under the action of goaf in the upper section of No.15401 Face of Huayang No.1 Mine, and to determine its reasonable size, theoretical analysis, numerical simulation and field monitoring research were comprehensively carried out. Based on the theory of ' internal and external stress field ', an arc triangular block structure model of overlying strata above section coal pillar was established. The calculation expressions of internal stress field range, coal plastic zone width and residual bearing zone were derived, and the balance relationship between coal pillar support and overburden load was further obtained. Then, a two-dimensional discrete element model was constructed by FLAC3D, and the plastic zone expansion, vertical stress transfer and roof subsidence law under different coal pillar width conditions were compared and studied. The results showed that when the width of the coal pillar increased from 14 m to 20 m, the plastic zone gradually shrank from the through failure to the edge area, and its proportion decreased from about 70 % to less than 30 %, and a stable elastic core zone was gradually formed in the middle of the coal pillar. In addition, the field measurement was carried out by arranging the borehole stress meter at the depth of 2~10 m in the coal pillar. The monitoring curve was consistent with the simulation law, and the 20 m wide coal pillar showed good bearing stability. The research results could provide technical basis for the layout of gob-side entry and the optimization of section coal pillar width.