Abstract: In order to ensure the fully mechanized top coal caving mining face not affected by the Tertiary System Aquifer in the overburden strata, to determine the rational thickness of the left safety coal (rock) pillar shall understand the failure law of the Cretaceous overburden strata under the conditions of the fully mechanized to p coal caving mining.Based on the site measured results of the failure height in the overburden strata and in combination with the lab similar material simulation, the rat io relationship between the failure "two zone" height of the Cretaceous overburden strata and the mining height under the condition of the fully mechanized top coal cavi ng mining was obtained.The failing and mining ratio of 5.61 and the cracking and mining ratio of 12.21 was applied to be the numerical basis of the rational left safety C oal (rock) pillar under the fully mechanized top coal caving mining in the top slice of No.7 seam.In comparison analysis on the related failure data of the overburden stra ta with the similar mining condition in the mine, the paper held that the overburden failure in the Cretaceous strata would have small cracking and mining ratio and the I arge falling and mining ratio features and could meet the failure law of the soft and weak type overburden strata.