Abstract: Steep inclined coal seam is threatened by water-rich layer, so whether the safety mining can be realized or not depends on the effective waterproof ability of floor aquifuge which is related to floor failure characteristic and depth. Based on the analysis of the law of floor damage, we predicted the depth of floor damage by elastic-plastic mechanics theory and divided floor aquifuge into regions by water bursting coefficient. In addition, we also carried on the technical and economic analysis based on recoverable resources and got the conclusion of mining profit and loss value.