Abstract: In order to define the water-proof coal pillar width of mining border between Yangcun Mine and Tianzhuang Mine, this paper proposed that a part of coal pillar was replaced by concrete water-proof partition pillar, constituting the boundary water-proof structure composed of narrow coal pillar and concrete water-proof parti tion pillar.This paper analyzed condition of the combined structure and pure coal pillar structure under underground pressure and water pressure by FLAC3D software, a nd studied critical size of pillar for resisting water effectively.The result showed that the width of coal pillar was 2.5 m and the width of concrete water-proof partition wa s 2.5 m for the boundary water-proof structure composed of narrow coal pillar and concrete water-proof partition.The maximum vertical pressure wasin the 1.5 m depth of the coal pillar beside the goaf and 2.3 times larger than original rock stress in pure coal pillar structure. Elastic area width of the pure coal pillar for resisting water eff ectively is 1.0 m, agreeing with the theoretic result. The maximum vertical pressure was in the 0.5 m depth of the concrete water-proof partition pillar and 3.2 times larg er than original rock stress in combined structure composed of the narrow coal pillar and concrete water-proof partition pillar.Elastic area width of the structure was 3 m, . providing higher security.