Abstract: During the coal mining in ultra thick seam under water body in Binchang Mining Area,in order to prevent water disaster accident occurred when caving zone and zone above coal mining face connected with aquifer of Luohe Formation,based on the study on ratio between cracking height and mining thickness of sevelal mines in mining areas, the' under three objects' coal mining regulations and height calculation formula of water flow fractureabed zone above fully mechanized top coal mining face W ere applied to individually calculate and predict the height of the max water flow crack zone in the mining area and to study and determine the mineable thickness of seam i n mining area and the suitable pre- mining top slicing fully mechanized top coal caving mining or one passing full seam fully- mechanized top coal caving mining. The result S showed that when the fully- mechanized top coal mining technology with a pre- mining of top slice applied to realize a safety mining under the water body,in order to ensu re the low slice mining thickness not over 7. 5 m and the top slice mining height not over 3. 5 m. When one passing full seam fully mechanized top coal mining technology applied,the coal mining height could be determined according to distance between main mining seam and Luohe Formation in mining area with the ratio between the crack zone height and the mining thickness not over 20,the mining height should be seriously controlled. An interval top coal caving method should be applied to prevent the over - caving of top coal. During the coal production period,the roof control should be enhanced and the analysis on mine strata pressure monitoring and measuring should be e nhanced to ensure safety mining of coal mining face under water body.