There are 6.3million tons coal under the water body of Jianghe river witch belongs to Yuwu coal company. The coal thickness is from 4.0to 6.2meters, therefore, it is difficult to mine with safety under water body that may cause water disaster after mining by using the empirical formula of water flowing fractured zone, in order to determine the upper limit of water flowing fractured zone, underground upward slant hole zonal water injection test and upward slant hole visual method boreholes television test are used in 9boreholes to observe development height of difference mining thickness in study zone be carried out. Furthermore, inflow of water monitoring and water quality studied by comparison method and simulations are also carried out. The growth height under the condition of different mining heights are further studied. The results of observation and monitoring and simulation show that the height of water flowing fractured zone is mainly determined by coal thickness after mining, reliable data is provided for prevention and control of water in coal mining under water body, application example is provided for coal mining under water body by three faces success mining.