TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The catalyst is p-toluenesulfonic acid, methanesulfonic acid, or sulfuric acid.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The organic solvent N,N-dimethylformamide, N,N-dimethyl acetamide, dimethyl sulfoxide, or N-methyl pyrrolidone. The evocating agent is azodiisobutyronitrile, dibenzoyl peroxide, tert-butyl peroxide, di-tert-butyl peroxide, tert-butyl peroxypivalate, or tert-butyl-2-ethyl peroxyhexanoate.    TECHNOLOGY FOCUS - POLYMERS - Preparation: The method for preparing the crosslinkable polyimide based composite proton exchange membrane of semi-interpenetrating network type, comprises: (a) dipping the perfluorosulfonic acid resin into sodium hydroxide solution, taking it out after 22-26 hours and drying it until its weight is constant; dissolving the resin in organic solvent at 60-80 degrees C to form even-concentration solution; where 0.5-5 g perfluorosulfonic acid resin is added to 50 ml of the organic solvent; (b) dissolving the polyimide with an unsaturated bond into the organic solvent, adding the crosslinker and the evocating agent to the solvent and mixing with perfluorosulfonic acid resin at room temperature, obtaining even solution; where the mass ratio of the polyimide to the perfluorosulfonic acid resin is 1:9 to 1:1; the mol ratio of the vinyl crosslinker to the polyimide is 1:9 to 1:1; and the mol ratio of the evocating agent to the polyimide is 0.05:1 to 1:1; (c) forming the mixed solution obtained in (b) into the membrane on a polyfluortetraethylene plate by pouring, and drying it in a vacuum drying cabinet for 20-28 hours at 80-90 degrees C, obtaining the semi-interpenetrating network polyimide/perfluorosulfonic acid resin composite membrane of the sodium salt type; and (d) dipping the composite membrane obtained in (c) into dilute hydrochloric acid with a concentration of 10% for 24 hours prior to taking it out for washing; drying it until its weight is constant, and obtaining the desired product. Preferred Components: The polyimide contains a fluorine-contained or fluorine-free structure, and can be provided with a reactive group. The perfluorosulfonic acid resin is a polymer having a molecular chain containing no carbon-hydrogen (C-H) bond but only carbon-fluoride (C-F), carbon-carbon (C-C) bonds, and a sulfonic acid group. The crosslinker is a vinyl monomer or a double-functional group monomer capable of reacting with the reactive group in the polyimide.