Abstract: In order to improve the stability of the wall rock around the goaf-side driving roadway and the utilization rate of coal resources, Tashan Coal Mine of Datong mining area was taken as the research object, the reasonable width of segment coal-pillar for the goaf-side driving in fully mechanized caving face of ultra-thick coal seam was studied in this paper. At first, the theoretical analysis of reasonable width of segment coal pillar was carried out, then the mechanical models of goaf-side segment coal pillars in different widths were established by numerical software FLAC^3D. The results of numerical stimulation showed that when the width of segment coal-pillar was from 5 m to 10 m, the roadway was in the low stress area and the support would be relatively stable. Finally, combined theoretical calculations with numerical simulation, referring to the ground pressure law and the results of micro-seismic monitoring in fully mechanized caving face of Tashan Coal Mine, considering the roadway span, isolated goaf gas and other safety factors, it was determined that the reasonable width of segment coal pillar was 8 m.