Abstract: In order to grasp the evolution law of the mechanical characteristics of the floor under the disturbance of the first coal seam mining in Pingmei No.8 coal mine, this paper uses COMSOL Multiphysics software to simulate the stress distribution characteristics of the underlying coal rock layer under the disturbance of Ji15 coal seam mining. Results show that: under the disturbance of first mining layer, the vertical stress field of underlying coal stratum is symmetrically distributed in the middle of working face. Obvious stress-relief zone could be seen above and below the working face of first mining layer. There is stress concentration at both ends of the mining area, and the maximum stress concentration reaches 2.56. The stress-relief effect is within 30 m from the bottom of first mining layer. Meanwhile, the stress releasing rate could reach more than 95%, and the length of sufficient stress-relief zone could reach 70.22% of the length of working face in first mining layer. Therefore, the coal seam within 30 m of stress-relief layer should be preferentially selected as the first mining layer.