Abstract: Taking Huainan Pansan Coal Mine nearly horizontal seam under the protective layer mining projects as research object, based on the gas flow, the basic theory of coal and rock mass formation and finite element method, by simulating mining stress field and deformation field development process of the protected seam after mining the under protective layer, the paper identifies stress deformation and expansion deformation laws of the protected seam with the face advancing. Based on protection criteria of stress relief and coal deformation, the reasonable protection for border demarcation basis is put forward. The study have shown that under the situation of combining ground well drainage，after the nearly horizontal coal seam under the protective layer mining, the theory protect scope of the protected seam exists the expand space, which can be extended to the same long and wide with next layer of protection. Field test and numerical simulation results are basically consistent.