Abstract: In order to prevent and contral the roof water,to leave the water prevention coal pillars,rationally set the height of the "three zones" in the roof,taking No. 8 seam in Chengdi Mine of Xishan Town as a study object,an underground mine water injection leakage measuring method with overhead borehole and a transient elect romagnetic method were applied to measure the height of the water flow crack zone.The FLAC3D software was applied to the numerical simulation on the roof falling of the overburden strata and the strata migration law.The results showed that the water flow crack zone height of the numerical simulation and the site measurement was basically the same.The water flow crack zone height was set as 57 .58 m,the falling zone height was 16.32 m and the crack zone height was 41.26 m.Thus the statistic e xperience formula of the water flow crack zone stipulated in Mine Safety Regulations would be not suitable for the mining conditions of the fully mechanized one passin g full seam mining face in the medium thick seam.