Abstract: Aiming at the problem of mining No.3 coal under plash caused by surface subsidence of a coal mine in Shandong,the paper analyzes the amount of surface subsidence caused by coal mining.The development height of "two belts" in No.3 coal mine is measured by using" double end water blocking device" technology.Compared with the theoretical values and UDEC simulation values,the height of water flowing fractured zone is determined as 47.75 m,the safety height of waterproof coal pillar in No.3 coal mine is 70 m.Using FLAC software,the paper simulates the water threat degrees of different gas faults in the remaining different coal pillar widths.The results show that the coal mine can be carried out safety mining under water area.