A novel experimental route to fabricate porous polyimide membranes (PPMs) with ideal air permeability was reported. The polymer solution layer consisting of the corresponding polyamic acid (PAA), solvent, and dibutyl phthalate (DBP) with a boiling temperature of 340℃ was treated by a simple process. The polymer solution layer was first treated at a lower temperature (about 150℃), then the received solid membrane was further imidized at a higher temperature (about 270℃), and finally, DBP was removed from the membrane at a temperature above its boiling temperature. The final asymmetric polyimide membrane with a dense skin layer was obtained. To improve the air permeability of the polyimide membranes, the polymer solution layer was treated between two substrates. And the PPMs with open pores on both sides are fabricated and the air permeability of the films is improved greatly.