Abstract: In order to solve the technical problem of the bed separation water prevention and control under the condition of the fully-mechanized top coal caving mining in the ultra thick seam, based on several bed separation water inrush accidents occurred in mining process of fully-mechanized top coal caving mining face in ultra thick seam of Guojiahe Mine as a case, with mine strata behavior observation in coal mining face, the height observation of water flowing fractured zone in failure overburden, the water quality test analysis, the geological condition analysis of coal mining face, the analysis on overburden mechanics model, the numerical model analysis and other several means were applied to study on the influence factors of bed separation water inrush and the water inrush mechanism and effective measures of bed separation water prevention and control technology were provided. The study results showed that bed separation water inrush from fully-mechanized top coal caving mining face in the ultra thick seam was occurred by comprehensive roles of periodical mine strata pressure, syncline structure and other several factors. The paper provided three major bed separation water prevention and control measures of failure “two zones” height observations in overburden, roof watery detection and water drainages from watery zone and bed separation water. The implementations on rich water pre-drainage from fold structure zone in the fold and other structure zone, the tightness destruction of bed separation space, the water drainage from bed separation space and other prevention and control measures would establish bed separation water detection, prevention and control integrated technology system.