NOVELTY - Preparing a nano particle doped gel polymer film comprises: adding emulsifying agent with de-ionized water, adding a mixture of methyl methacrylate or butyl acrylate, reacting, adding initiator, and stirring and reacting to obtain a polymer emulsion; adding the emulsion with polymer latex, and stirring; washing the obtained product, and drying to obtain poly (methyl methacrylate-b-butyl acrylate); dissolving nano-particles in solvent, performing an ultrasonic treatment, stirring, coating on a supporting body of a gel polymer film, dipping into the water to prepare a nano-particle doped film.    USE - The method is useful for preparing a nano-particle doped gel polymer film for a lithium ion battery (claimed).    ADVANTAGE - The polymer film has excellent liquid-absorbing and liquid-retaining capacity, improved mechanical strength and thermal stability, good compatibility with the lithium, and is prepared in a simple and cost-effective manner with high production efficiency.    DETAILED DESCRIPTION - Preparing a doped nano particle comprises: adding 3 pts. wt. of emulsifying agent with 60-70 pts. wt. of de-ionized water under the protection of protective gas, adding 30-35 pts. wt. of a mixture of methyl methacrylate or butyl acrylate, reacting at 58-65 degrees C for 0.3-0.5 hours, adding 0.15-0.4 pts. wt. of initiator, and stirring and reacting for 6-10 hours to obtain a polymer emulsion; adding the emulsion with 1-1.5 times of polymer latex, and stirring; washing the obtained product with de-ionized water and absolute ethyl alcohol, and drying to obtain poly (methyl methacrylate-b-butyl acrylate); dissolving nano-particles in 90-96 pts. wt. of N,N dimethylformamide solvent, performing an ultrasonic treatment at 60-75 degrees C for 20-35 minutes, adding 4-10 pts. wt. of poly(methyl methacrylate-b-butyl acrylate), stirring until it is dissolved completely, coating on a supporting body of a gel polymer film of a lithium ion battery, dipping into the de-ionized water to obtain a milky white thin film, and continuously dipping for 2-3 hours to prepare the doped nano-particles; and drying the lithium gel polymer film, and cutting to the needed size to obtain a nano-particle doped gel polymer film for the lithium ion battery.