Abstract: The paper probed the roof destroyed altitude by pouring water into borehole method after the coal stratum exploited in fully mechanized sub-level caving face in view of the situation in Yangcun Coal Mine, Ascertained the lying concealed small faults in the working face and their extension condition by underground radio-wave penetration method, Proved the thickness on shallow bedrock roof of No.305 face; On this basis, Designed dimension of the sand-preventing coal pillars, lifted the upper mining limit to 30m, Worked out one testing scheme for fully mechanized sub-level caving face. Have got success of improving the upper mining limit in Fully-Mechanized Caving Face in No.3 coal seam by the experiment.