After analyzing fully-mechanized sub-level caving technology in the case of special thick and hard coal seam，it is presented that the increment of cavability of hard roof coal and available control of gas are the main difficult technology problem to be faced. The structure and breaking characteristic of hard roof coal are analysed，and the main technology approach and measurements to increase cavability of hard roof coal are proposed. The technology has been successfully applied in Baijigou colliery，Ningxia，where the coal seam is thicker than 20 m，with hard coal，hard roof，intensive gas and shallow oveburden. The annual output of coal there is 1.5 million tons. The proposed technigue is a breakthrough to sub-level caving mining technique，and anticipative effect has been obtained.