Abstract: To solve the difficult problem of determining the width of small coal pillar in gob-side roadway of fully-mechanized caving in extra-thick coal seam, taking 8305 fully-mechanized caving face of Tongxin Coal Mine as the research background, according to the theory of stratum movement, using the movement characteristics of stratum in triangle slip area, the distribution law of lateral support pressure before and after gob stability is analyzed, and the rock mass breakage in triangle area is obtained. The reduction of the force on the small coal pillar before and after the fracture is the fundamental reason for the decrease of lateral support pressure. At the same time, theoretical analysis and numerical simulation are used to obtain that the width of stress reduction zone is 17 m to 21 m after the goaf is stable. The maximum width of small coal pillar in gob-side roadway of fully mechanized top-coal caving in extra-thick seam is determined to be 11.5 m, and the field test is carried out by means of rock pressure observation. The results show that the deformation of roadway is small, the setting of small coal pillar is reasonable, the maximum width of small coal pillar is 11.5 m, which can meet the needs of safety production in working face, and the coal recovery rate is increased.