Abstract：In condition of thick seam and large mining height，the step instability of rock covering is caused by height of the caving zone；and fractured zone is increased and activity degree of strata is enhanced in fractured zone. Therefore，it is the important factor that causes water inrush in mining. However，the step instability of stratum in fractured zone is related to lithologic structure and combination of lower strata. Based on the important influence of lithologic structure on step instability and the water inrush prevention under large and thick seam mining height，the law that different lithologic associations among coal seams with influences on integrity and continuity of upper coal seam are studied by numerical calculation and field test. The conclusions show，when lithologic association between coal seams is soft-hard，as the proportion of lower strata to spacing of layers is higher，the step generated in upper coal seam is lower，and soft strata structure is favorable for decreasing step of upper coal seam than hard strata structure. Components of lithologic structures are put forward to determine step values；and the step and distribution law of cracks are determined. The effective measures are presented to prevent the water inrush；and safety production of working face is guaranteed.
