Abstract: In order to study the reasonable width of coal pillar in gob side entry of fully mechanized top coal caving in extra thick coal seam, the 8305 working face is taken as the research object, and the method of theoretical analysis, numerical simulation and field practice is adopted. The reasonable width of coal pillar is 5.4-9.9 m by the internal and external stress field theory, limit equilibrium theory and the support conditions of coal pillar side in gob side entry. The reasonable width of coal pillar is determined to be 6 m by the distribution law of horizontal zero displacement surface and vertical stress peak value with the numerical simulation, and the targeted surrounding rock control technology is proposed. The practice shows that the displacement of roof and floor, the deformation of coal rib and coal pillar are small, and the surrounding rock control effect is well.