Abstract:Through engineering geology investigation and theoretical analysis,the mechanism of abnormal water inrush in the roof of working face 745 under the condition of giant thick igneous rock cover in Haizi mine is researched. The results show that the water accumulation in the separation area under the giant thick igneous rock cover with 62 m away from the coal seam is the water source that causes the abnormal water inrush of working face 745. According to the traditional estimation method,the height of water flowing fracture zone is 25.3–36.5 m,which can not connect the accumulated water in the separation area under the giant thick igneous rock. However,the loading transfer effect of separation area water induces compound breakage of two inferior key strata in lower part,so height of water flowing fracture zone develops abnormally,and it connects separation area water,which is the reason of the 745 working face abnormal water inrush. The mechanism of separation area water inrush indicates that probing and discharging borehole drilled to the separation area can prevent this kind of water inrush.