NOVELTY - A composite membrane comprises porous membrane prepared from raw material of organic polymer resin as a matrix, and dense membrane prepared from cation exchange resin, on one side surface of the matrix. Also claimed is, preparation of composite membrane, which involves dissolving 5-70 wt.% organic polymer resin in solvent, pouring the resultant mixed solution on nonwoven fabric substrate, maintaining for 0-2 minutes, dipping the resultant material in poor solvent for 5-600 seconds, forming film having thickness of 20-500 mu m, coating the solution (A) on the film surface and drying.    USE - Composite membrane is used in vanadium redox flow battery, zinc/bromine redox flow battery, polysulfide/bromine redox flow battery, iron/chromium redox flow battery, vanadium/bromine redox flow battery and zinc/cerium redox flow battery (all claimed).    ADVANTAGE - The composite membrane has excellent hydrophilicity and ionic selectivity, can effectively inhibit electrolyte solution migration problem, and is prepared by simple and environmentally-friendly process.    DETAILED DESCRIPTION - A composite membrane comprises porous membrane prepared from raw material of organic polymer resin as a matrix, and dense membrane prepared from cation exchange resin, on one side surface of the matrix. The porous membrane has aperture size of 0.1-20 nm and porosity of 6-70%. Also claimed is, preparation of composite membrane, which involves dissolving 5-70 wt.% organic polymer resin in 0-50 wt.% solvent chosen from dimethyl sulfoxide, dimethylacetamide, N-methylpyrrolidinone and dimethylformamide, mixing at 20-100 degrees C for 0.5-10 hours, pouring the resultant mixed solution on nonwoven fabric substrate or directly pouring on flat plate, maintaining for 0-2 minutes, dipping the resultant material in poor solvent for 5-600 seconds, forming film having thickness of 20-500 mu m, spraying, dipping or spin coating the solution (A) on the film surface, drying at 0-200 degrees C for 1-24 hours and obtaining composite dense membrane having thickness of 0.1-100 mu m. The solution (A) is obtained by mixing 0.1-10 wt.% cation exchange resin in solvent chosen from dimethyl sulfoxide, dimethylacetamide, N-methylpyrrolidinone and dimethylformamide. The solvent used for dissolving organic polymer resin is optionally mixed with volatile solvent.