NOVELTY - Preparation of microporous structured polymer film comprises (1) preparing the foaming agent: mixing the carbonate with organic acid, milling it uniformly to obtain the foaming agent; (2) preparing solid polymer film: dissolving the polymer in solvent to obtain polymer solution, and adding the complex foaming agent into the polymer solution, volatilizing the solvent to obtain the solid polymer film; and (3) foaming it: placing the solid polymer film in microwave reactor and heating to obtain the polymer film with microporous structure.    USE - The process is useful for preparing a microporous polymer film which is useful in serving as polymer electrolytes in the lithium ion secondary battery (all claimed).    ADVANTAGE - Preparation of microporous structured polymer film has strong operability and the obtained product has steady quality. The complex polymer can be polymer electrolytes of lithium ion secondary battery with excellent ion electrical conductivity and mechanical property. Moreover,    DETAILED DESCRIPTION - Preparation of microporous structured polymer film comprises (1) preparing the foaming agent: mixing the carbonate with organic acid in mass ratio of 20:1-1:20, milling it uniformly to obtain the foaming agent; (2) preparing solid polymer film: dissolving the polymer in solvent to obtain polymer solution, and adding the complex foaming agent into the polymer solution at mass ratio of complex foaming agent to polymer 1:2-1:200, volatilizing the solvent to obtain the solid polymer film; and (3) foaming it: placing the solid polymer film in microwave reactor, heating it at 40-140 degrees C to react for 20-30 seconds to obtain the polymer film with microporous structure; where carbonate is carbonate or supercarbonate of alkali metals and alkaline-earth metals, and organic acids is citric acid, tartaric acid, glucuronic acid, mannuronic acid, formic acid, acetic acid, oxalic acid or glycine; polymer is polyethylene glycol oxide, polyacrylonitrile, polymethylmethacrylate, kynoar or kynoar-hexafluoropropylene polymer, or blending or copolymerization system of these polymers.