Abstract: According to wide application of Слесарев formula that have been written in Rule of Mine Water Hazard Prevention and related literatures and books in mine water hazard prevention, through the principle of materials mechanics, formulas of retaining width of waterproof coal pillar and advanced distance and water stop casing length of probing and discharging borehole are proved from the theory formula based on the mode of simply supported beam. The formulas of safety hydraulic head and aquiclude thickness are proved from the theory formula based on the mode of statically indeterminate beam. Because mechanical error exists in actual application of theory formula based on the mode of simply supported beam, in order to avoid water inrush due to the smaller L value, the necessity to increase safe coefficient K value and abide by Rule of Mine Water Hazard Prevention is emphasized. Because obvious regularity of ground pressure and non-uniform load exist in working face floor, Слесарев formula of safety hydraulic head and aquiclude thickness are only suitable for risk assessment of gateway water inrush.