In order to improve the design reliability of the porous asphalt pavement, a 3-D finite element seepage analysis method is used to evaluate the drainage capacity of the porous asphalt pavement, based on gradation criteria of short-term rainfall intensity. The results indicate that the drainage capacity of porous asphalt course (PAC) is improved effectively with the increase of the gradient of cross slops, the thickness of the course and the permeability coefficient of porous asphalt mixture. Moreover, the width of the porous asphalt layer also has a significant impact on the drainage capacity of PAC. The condition of no runoff of the porous surface layer under certain rainfall intensity can be confirmed by the results, which can guide the choice of the thickness and width of the porous asphalt layer and the determination of the aim void of the porous asphalt mixture.