Abstract: Based on the characteristics of the mine roof,floor strata composition and structural properties of the working face in a coal mine of Yanzhou,established the engineering geological model to reflect the complete floor strata combination.Analyzed the floor stress and distribution characteristics of plastic zone by FLAC^3D numerical simulation in the process of coal seam mining,got the depth of deformation and failure of mining coal seam floor.Finally,combined with the variation of the ultrasound imaging observations in the field seam floor drilling with different depths,educed the depth of failure of coal seam floor is about 12 m at this working face.