Abstract: In order to protect safety mining of No.7802 fully mechanized top coal caving mining face of Wuyang Mine under Zhanghe reservoir, the paper studied th e height development law of the water flow crack zone in the fully mechanized top coal caving mining, discussed the relationship between the overburden strata structur e failure and the mining permeability of the fully mechanized top coal caving mining face, calculated the height of the waterproof safety pillar required for the fully mech anized top coal caving mining under the reservoir and set up the related waterproof and control technical measures.The results showed that the development height of t he water flow crack zone in the medium and hard strata above the fully mechanized top coal caving mining face would be basically direct proportion to the one passing cutting height of the seam and the percentage coefficient was 20.22.The base rock thickness of the roof above the seam was higher than the height of the safety pillar r equired for the mining under the reservoir.The monitoring and nearing results showed that the water inrush in the coal mining face would have no relationship with the s urface ground water and the high production and high efficient safety mining under the reservoir could be realized.