Abstract: Taking Haizi coal mine as engineering background,and using FLAC^3Dsoftware,the paper simulates remote lower protective 10^# seam mining in extremely thick key layer and the mining without key layer.Then,through contrastive analysis,the influence of extremely thick key layer on pressure-relief effect of protected 9^# seam is obtained.The results indicate that pressure-relief effect in extremely thick key layer is better than that without key layer,which is the reason of pressure-relief and gas drainage by crossing drillings in long-distance overlying coal seam.After result investigation,the pressure of protected seam is released effectively.