Abstract: Based on the front and rear support pressures during the mining process of the coal mining face and in consideration of the water pressure of the aquifer under the floor, a mechanics model with the support pressure and water pressure jointly effected to the floor was established in order to calculate the stress at the any point of the floor.With the combination with the Mohr-Coulomb Criterion, the judgment basis of the floor failure or not could be provided.According to the geological cond itions of No. 1028 coal mining face in Suntuan Mine, the theoretical calculation showed that the strata within 020 m depth under the seam floor were highly affected by th e mining activities, the failure at 2 meter in the front of the gateway driving face would be developed toward the direction of the goaf, the failure depth scope would be w ithin 016 m and the max failure depth would be 16 m.The electric resistance rate test and measurement method was applied at the site to detect the floor failure conditi on at the different locations of the gateway driving face. The results and the theoretical calculation were closed to each other and could verify the correction of the theor etical analysis for the mechanics model.