Abstract: The current " three under " mining rule regulates that the boundary angle of ground subsidence basin is determined by taking subsidence of 10 mm as boundary points,and the boundary angle is from 50 ° to 65 ° according to different overburden rock types.Surface subsidence measured data of thick unconsolidated layer mine and mining-induced damage example show that the existing method and the size of the angle value for determining the boundary angle are not very well to meet the needs of mining engineering.Amendment suggestion is put forward to determine the boundary angle by taking the horizontal subsidence of 10mm near the boundary zone of ground subsidence basin as boundary points in thick unconsolidated mining area,and the boundary angle value reduces 5 ° to 10 °on the basis of the current regulation.The suggestion has guiding significance and practical value to the efficient and safety production of the mine.