Disclosed is a method of preparing porous polybenzimidazole. The method includes providing polyaminoimide by reacting aromatic amine including at least two ortho-positioned amino groups and acid dianhydride, obtaining polypyrrolone from the polyaminoimide, subjecting the polypyrrolone to alkaline treatment, and subjecting the alkaline-treated polypyrrolone to heat treatment. The polybenzimidazole shows permeability and selectivity for various gases due to a fractional free volume and well-connected picopores.