Abstract: In order to ensure the safe production of high-gas mines with close coal seams, it is necessary to master the evolution law of porosity of coal and rock and the void fraction model of goaf after coal seam mining in order to provide theoretical basis for mine gas drainage. In this paper, taking the NO.3 and No.6 coal seam in Xiangsheng Coal Mine as the research background, numerical simulation software is used to simulate and analyze the porosity evolution law of coal and rock after downward mining of NO.3 and No.6 coal seam, and the void fraction model of No.6 coal seam goaf is explored. The results show that the areas where the porosity of coal and rock in NO.3 coal seam changes obviously are mainly distributed in the caving zone, and the porosity of coal and rock in No.6 coal seam first increases and then decreases. The void fraction distribution in the direction of strike, dip and height of the floor in goaf of No.6 coal seam is exponential function.