Abstract: The floor failure and slip is one of the key technologies to be resolved in steeply dipping seam mining. Based on geological conditions and characteristics of strata behaviors of No. 25221 working face in 2130 Coal Mine, the mechanism of floor failure and slip is investigated by theoretical analysis,numerical simulation and field monitoring. The results show that the floor developed from continuum state to block-fractured state under cyclic loading and unloa- ding. The failure pattern of the floor presents an asymmetric inverse arch along inclined direction of working face,the maximum damage depth is located at the lower part of the working face,and the floor damage depth and range decrease with the increase of the dip angle of the coal seam. And the impact of the support load on the failure of the floor is lim- ited,which mainly involved immediate floor. If the included angle between the resultant force of active force acting on the sliding body and the normal direction of slip surface is larger than the frictional angle of the slip surface,floor slip will be appear. And with the increases of coal seam dip,fragmentation degree of immediate floor and range and scope of inclination of support,the slip probability of floor increases.