Abstract: Based on the characterization of inrush of water in bed separation caused hard and thick overburden breakage, we analyzed the geological engineering conditions in a coal mine first mining, such as overlying strata spatial occurrence, rock mass structure, mechanical properties. The first mining face and the subsequent face overlying hard magmatite rock sill rupture caused by bed separation first closure distance and cycle closure distance was calculated and simulated by using thin plate theory and numerical simulation methods. The results showed, bed separation first closure distance is about 300 m and cycle closure distance is about 130 m;because subsequent work mining was effected by the adjacent mining face goaf, initial closure distance is about 260 m and cycle closure distance is about 120 m.The results are in accordance with the field case analysis. According to the bed separation closure rules, we proposed the water in bed separation disaster prevention measures. The achievements were applied to the working faces to guarantee mining safe.