Abstract: The paper describes a new method of failure depth of coal seam floor by using BOTDR(Brillouin Optical Time Domain Reflectometer) distributed optical fiber sensing technology. Taking a coal mine in Huainan as an example, a distributed fiber monitoring scheme of coal seam floor deformation was designed. By monitoring the deformation state and the strain distribution of the sensing optical cable buried in the floor of coal seam in the monitored space, combined with the consideration of coal seam floor strata structure, the distribution characteristic and the changes of deformation of coal seam floor during the process of the working face advancing are presented. The results show that, the BOTDR distributed optical fiber sensing technology can not only accurately reflect the deformation of coal seam floor but also monitor the whole process of progressive deformation and failure of the floor failure depth development. Thereafter, it's possible to determine the failure of the floor failure depth according to the strain distribution of sensing cables.