The engineering geological model and numerical simulation models for fully mechanized sub-level caving under thick unconsolidated water-bearing strata are constructed to predict and simulate the stress distribution and the extent of failure zones in the overlying rock mass. The distribution of the principal stresses in overburden rock mass is divided into three types. The failure conditions of overburden are classified into tensile cracked area, tensile-fissured area, and plastic deformation or shear fractured area. The extent and height of water flowing fractured zone can be determined on the stress distributions and failure conditions.Based on the calculation results and comprehensive engineering geological and hydrogeological evaluation, the height of safetypillars under water body of mining thick coal seam No.3 in Taiping coal mine is then defined to be 60m, which is the minimum size of pillars in similar geological and mining conditions in Yanzhou coalfield.