Abstract: To provide theory basis for safety mining of steep seam under water body，the UDEC2D numerical calculation is used in this paper to analyze the influence of key stratum in overburden on water flowing fracture distribution and waterproof coal pillar stability，based on the occurrence conditions of coal-rock seams and hydrogeology conditions in South No.2 District of Longhu coal mine．The results show that when there is no key stratum in overburden，the water flowing fracture in steep seam mining presents“ear style”distribution．When there exits key stratum in overburden，the water flowing fracture is mainly composed of bed separated fracture parallel to the bedding face，and will be parabolically increased in tendency with mining height．In addition，the water flowing fracture zone begins to develop upwards the key stratum and its upper strata after the first breakage of main roof．The exit of key stratum in steep seam can enlarge the falling range of waterproof pillar obviously，and the water body is more easily to enter into the goaf along the plastic failure zone of waterproof pillar．Hereby，the strip and up-dip backfilling mining method is designed，and the corresponding measures are adopted to reinforce the waterproof pillar，so as to ensure safety mining of steep coal seam under water body．

Key words: steep coal seam, key stratum, water flowing fracture, waterproof pillar, fluid-solid coupling