In this paper,the analytic mechanics model of fault activation and water inrush from floor is established base on the material mechanics and theory of coal mine stress.The correlative formulas of water pressure and mining depth for fault activation and water inrush from floor under the critical condition of water-inrush are drawn by comprehensively considering the stress conditions of fault plane and its nearby area and deducing the correlative formulas of fault activation combining water pressure.The relevant parameters of formulas are discussed.Finally,the formulas are used to the analyses of fault activation and water-inrush from floor in the 22081 working face of Chaohua mine in Zhouzheng mining area,the calculated results are approximate with the actual situation,this indicates that the analytic mechanics model and the relevant formulas can be used as one of the criteria fault activation in mining floor.