Abstract: Setting the height of the water conducting zone in roof can provide the basis for prevention and control of roof water, mining and excavation layout, leaving the water prevention coal pillars, design of coal gas drainage. Taking No.8 seam in Zhenchengdi mine, Xishan as a study object, using water injection leakage measurenment method in overhead borehole, the height was 57.98 m, the falling zone height was 16.72 m and the fractured zone height was 41.26 m. Based on observation result from 8 mines in the similar conditions, empirical formulas of water conducting zone height for fully mechanized mining face were obtained by statistical regression analysis. The contrastive analysis of the corresponding formula in "coal mine safety regulations" was carried out. The result show that the empirical formula has good applicability, formula in "coal mine safety regulations" has relatively large error when applied in prediction of the mining conditions for mining height of fully mechanized mining in median thick seam.