In order to guarantee the safety of the coal mine, needing to leave in reasonable waterproof safety coal pillar. Aiming at the working face 3311 of Xuchang coal mine, this article based on the analysis to the basis of factors for improving the mining limit, using UDEC numerical simulation, the regulations of coal mining under buildings, railways and water bodies and historical experience to analyze crack-production ratio of the working face, study the height of water flowing fractured zone development, designing test mining program. Through the upper mining limit to liberate 160000t coal resources and to gain the obvious economic benefits, which provide important significance for other similar coal mines in coal field of North China area.