NOVELTY - A proton exchange membrane preparing method involves cutting ethylene-tetrafluoroethylene copolymer film. Oil dirt on a surface of the block is removed, dried and weighed. The film is put into a polyethylene film bag, and the bag is sealed and placed at an electronic beam irradiation chamber dynamic radiation. The irradiated membrane is put into alpha-methylstyrene and methacrylic acid in a container, and the container is placed in 75 degrees C constant temperature water bath device. The grafted film is taken out, and washed by toluene. A sample is taken out, washed by water, and dried.    USE - Method for preparing proton exchange membrane for a fuel cell.    ADVANTAGE - The method enables providing operation convenience, reducing the cost, increasing water absorbing and water holding capacity, and improving proton conductivity, gas diffusion capability and property.    DETAILED DESCRIPTION - A proton exchange membrane preparing method involves cutting ethylene-tetrafluoroethylene copolymer film into a rectangular block. Oil dirt on a surface of the block is removed by utilizing acetone, dried and weighed. The film is put into a polyethylene film bag, and high purity nitrogen is charged to replace air. The bag is sealed and placed at an electronic beam irradiation chamber dynamic radiation, where the electronic beam intensity is 1 mA, and the total irradiation dose is 150 kiloGray. The irradiated membrane is put into alpha-methylstyrene and methacrylic acid in a container, and toluene is mixed with nitrogen and oxygen. The container is placed in 75 degrees C constant temperature water bath device for grafting reaction mixed solution. The grafted film is taken out from the container, and washed by toluene. The surface of a sample homopolymer is removed, and put into oven for 50 degrees C. Sulfonation reaction is performed by utilizing sulfonic acid as a sulfonating agent, and the sample is taken out and washed by water. The sample is dried to obtain a proton exchange membrane of anti-oxidative degradation for a fuel cell.