Determination of waterproof coal pillar under aquifer was closely related to whole safety mining. In order to determine overburden waterproof coal pillar height after No.5 layer mining， formula and empirical formula recorded in《Sage Rules》as well as UDEC numerical simulation method were used to analyze overlying rock fracture height. Maximum height of water flowing fractured zone was 39.6 m through theoretical calculation and the fracture zone height was 32 m through numerical simulation. Finally， 39.6 m was selected as height of water flowing fracture for  safety reasons. Based on calculation of empirical formula in regulation， we got that waterproof coal pillar height was 52.86 m and the height of waterproof coal pillar in actual production was 53 m.