Abstract: With the increase of mining depth, Huaheng Coal Mine is severely threatened by water inrush at floor limestone. Aimed at faults which are in charge of water inrush, the fault influencing factor and fractal dimension theory are introduced to quantitatively study the structure characteristics of coal seam floor. According to water inrush cases in Xinwen Coal Mine, scatter diagram method and grey correlation analysis are adapted to analyze the correlation between faults and water inrush. As a consequence, there is a nonlinear positive correlation between fault complexity degree and water inrush danger degree and fault influencing factor is the main controlling factor while fault fractal dimension value is secondary. With elements above, fault controlling index is proposed to estimate the complexity degree of fault, floor damage degree caused by structure and water inrush danger degree. By analyzing and calculating, 0.7, a critical value of fault controlling index in Huaheng Coal Mine is presented and used to divide Huaheng Coal Mine into safety zone and water inrush risk zone.