To solve the serious problem of rib spalling with “three soft”thick coal seams under the condition of fully mechanized caving in Shajihai Mine, the methods of theoretical analysis and field monitoring was used. The stress characteristics of coal wall were studied and the mechanical model was established to analyze the spalling mechanism. The key factors affecting rib spalling were put forward, the control countermeasures were determinate and carried out. The results showed that the ratio of the anti shear force along the shear sliding surface to the sliding force on the sliding surface was less than 1, and the coal wall was prone to shear failure, resulting in the phenomenon of coal wall spalling. It was proposed that the key to control the rib spalling was to determine the reasonable coal cutting height, support working resistance and propulsion speed. The on-site coal wall detection results of No. B1003W03 Face showed that this countermeasure could effectively improve the caving situation of working face and had a good application effect.