Abstract: Numerical simulation was conducted for hydraulic fracturing process in coal seam according to solid-liquid coupling equations and Mohr-Coulomb principle. The evolution of seepage field, stress field and fracture field (three fields) during process of crack propagation in coal seam was analyzed respectively. The results show that the three fields are changing dynamically during the process of hydraulic fracturing. The fracture is motivated to expand by pore water pressure, as a result of which, crack damage increases, seepage field centering to the fracture expands gradually, and high stress area and low stress area are developed from original stress field, and two regimes of low stress and high permeability field in a vertical direction of main fracture are ultimately formed. All of the above contribute to an effect of pressure relief and permeability improvement.