The present invention relates to a method for preparing a porous polyimide film, comprising reacting an aromatic dianhydride with one or more aromatic diamines in a suitable solvent to form poly(amic acid), adding a dehydrated agent of an acid anhydride and an organic base to the reaction mixture to convert the poly(amic acid) to a polyimide precursor, casting the reaction mixture comprising the polyimide precursor onto a solid support to form a film, coagulating the polyimide precursor in a coagulating bath comprising a mixture of a solvent and a non-solvent to develop a porous structure, and drying the coagulated polyimide precursor in air to form the porous polyimide film. A composite membrane comprising same and its use are also provided.