By analysis the form of gas percolation in coalfield fire, the follow conclusion is obtained : under the influence of natural wind pressure and fire pressure, natural ventilation system of coalfield outcrop is formed automatically during coal spontaneous combustion, which exists in two forms, one is the fresh airflow percolation in caving zone and its pore, the other is coal combustion gases migration movements in cracks after sliding. The fluent numerical simulations show that natural ventilation system in coalfield outcrop fire district is a vacuum ventilation system; From inlet to combustion area air intake line, air pressure and gas percolation on linear relationship, and gas percolation meets Darcy law in the porous medium percolation theory.