In order to analyze the void reduction of porous asphalt mixture after loading, the discrete element software PFC2D was used to establish the two dimensional digital specimen of porous asphalt mixture and virtual rutting test. New void generation procedure was developed, void graded in asphalt mortar was simulated as irregular polygons which were put into asphlt mortar and then deleted so that void was formed. The microscopic parameters of porous asphalt mixture were obtained by the static creep test. Loading modes and gap information extraction algorithm of the virtual rutting test were designed. The feasibility of virtual rutting test was verified by the comparison with lab rutting test. Through series of virtual tests under different factors, it is showed that void reduction increases as the loading increases; the rate of reduction slows down gradually to a stable state before the structure damage; temperature has great effects on the void reduction, and as the temperature increases, three stages of change is shown; the void reduction reaches bottom when the initial void ratio(by volume) is 20%.