Abstract: To mine coal resources threatened by confined groundwater in seam floor in North China coal fields, it is necessary to understand the destruction regularities in surrounding rocks of the floor. In this paper, by the nonlinear three-dimensional computer laboratory simulation we studied deeply the stress distribution in seam floor and found that there is the cross-layer arc in the mining floor, pointed out that the cross-layer arc is a bearing structure of groundwater and water pressure. Through the theoretical calculation, we have obtained the biggest principal stress formula of cross-layer arch, according to the formula we got the position where the fractures occur in seam floor, providing the reference for the design of the width of coal face in coal mines threatened by the confined groundwater in North China coal fields.