Abstract: Based on in situ measurements, similar material simulation experiments and theoretical analysis,this paper emphatically analyzes the rock failure of the floor in coal seams caused by changing the dimension of the mould.The relation between the limited contact pressure and the mould dimension: =aΦ b ,and other conclusions are obtained.The new concepts “the secondary failure”and “the equal depth contact pressure”etc.are helpful to understanding the rules of indentation failure of the floor rockmass .