Abstract: The distribution of porosity in the goaf has an important influence on the fluid flow in the goaf. Based on the O-shape ring theory and the masonry beam theory, a three-dimensional distribution function of porosity was proposed. Based on the 3D porosity model, the effect of gravity and the average particle diameter on the gas migration and concentration in the mining area was simulated and studied in Pingshu 15111 fully mechanized mining face. The gas migration and enrichment regularity with gravity and different average particle diameter values in the caving area are studied. The results showed that under the consideration of gravity, the gas in the mining area formed an obvious “downward tie” effect at the upper corner, and made the gas accumulate at the upper corner. The average particle diameter in the mining area increased, and the low concentration gas enrichment area expanded, and the high concentration gas enrichment area decreased. By comparing the measured data, it was found that, the particle diameter setting of 0.12 m under the consideration of gravity is more realistic.