Abstract: The void parameters such as void radius and void space are achieved by image analysis of porous asphalt mixture sample slice through cosmos image analyzing software,so as to set up the grid sizing of the permeability model of the porous asphalt mixture and the input of original value. The method of Monte Carlo is used to set up the permeability model. The mathematics model abstracted is based on the Hagen-Poiseuille theory and the flab iterative method. The result shows that the numerical simulation permeability outcome is consistent with the indoor permeability test outcome. The model can simulate any maximum congregate diameter at different water surface levels and the permeability coefficient of any open-graded porous asphalt mixture. The model offers an effective method for designing of the porous asphalt mixture.