Abstract: Fissure field in goaf of coal mine is the main site of gas disasters and fires. To analyze the inside gas flow law, realistic goaf porosity must be obtained first. Based on the analysis of the different distribution characteristics of “horizontal three zones” and “vertical three zones” of overlying strata fissures in goaf, combined with “O” ring and rock beam theory, caved zone is analyzed and obtained with distribution of bulking factor and distribution function of subsidence of fractured zone, a three-dimensional distribution model of porosity in goaf is established. The applicability of the model is verified through actual cases, and porosity in caved zone is 0.1 to 0.43 and fractured zone is obtained. The porosity reaches the maximum value of 0.31 at a distance of 20 m to 30 m from the bottom plate. The results show that porosity in caved zone is distributed in a “hovel” shape under the influence of bulking factor. From compaction zone along x, y, and z axes to boundary, its value gradually increases from the minimum; in fractured zone, affected by the amount of subsidence in the regular moving zone and the length of the broken rock, the development of the separation layer is large in the surrounding area and small in the middle, and porosity is in a “double hump” shape, and the “bump” is 30 m to 50 m away from the coal face and coal pillar. The porosity is large, as the buried depth becomes shallower, the maximum porosity is closer to the middle in goaf, the basic stability line increases, and the shape of fissures conforms to the distribution of the “O” ring.