NOVELTY - A fluororesin is obtained by copolymerization of tetrafluoroethylene, ethylene, and perfluorosulfonyl monomer (I). The fluororesin has molecular weight of 200000-600000 at 300 degrees C and load of 5 kg, and melt flow index of 1-100 g/minute.    USE - Fluororesin is used for fluorine-containing polymer fiber for forming fiber sheet used as proton exchanging membrane of fuel battery and skeleton reinforced material for chloro-alkali ion composite membrane (all claimed).    ADVANTAGE - The fluororesin has excellent ion exchanging ability, and provides fiber sheet having excellent adhesiveness and strength.    DETAILED DESCRIPTION - A fluororesin is obtained by copolymerization of tetrafluoroethylene, ethylene, and perfluorosulfonyl monomer of formula: A-(CF2)a-(CF(-Rf")b)-CF(-Rf')-O-(CF(-CF3)-CF2-O)n-CF=CF2 (I). The fluororesin has molecular weight of 200000-600000 at 300 degrees C and load of 5 kg, and melt flow index of 1-100 g/minute.    a,b=0-5, with sum of a and b being not equal to 0;    n=0-2;    Rf",Rf'=-F, -Cl, -Br or -CzF2z+1;    z=1-5;    A=-SO2F, -SO2Cl, -SO2Br, or -SO2NR1R2;    R1,R2=-H or -CmH2m+1;and    m=1-10.    INDEPENDENT CLAIMS are included for the following:    (1) fluorine-containing polymer fiber, which is obtained by melting and extruding the fluororesin at 200-400 degrees C using melting extruding equipment, spinning, drawing and cooling;    (2) preparation of solution of fluororesin, which involves cleaning a reaction kettle and drying, vacuuming, supplying nitrogen gas to replace air, controlling the content of oxygen to less than 10 ppm, vacuuming to less than -0.1 MPa, adding perfluorocarbon, and perfluorosulfonyl monomer (I) into the reaction kettle, blending and controlling temperature to 35-45 degrees C, adding perfluoroperoxide initiator by metering pump, after the temperature of the system is constant, and introducing mixed gas of tetrafluoroethylene and ethylene to allow pressure to reach 1 MPa, continuously introducing the mixed gas of tetrafluoroethylene and ethylene to maintain the reaction pressure at 0.8-1.0 MPa, to enable continuous reaction, discontinuously adding perfluoroperoxide initiator into the system by metering pump, and when the adding amount of mixed gas of tetrafluoroethylene and ethylene reaches 1000-1200 g, stopping addition, and continuing the reaction, and when the pressure of the kettle is reduced to 0.8 MPa, stopping reaction, recovering the unreacted mixed monomer into a recovering tank, discharging the material into a harrow-type dryer by discharge valve of the reaction kettle, recovering the reaction solvent and unreacted sulfonyl monomer, and drying the polymer material and obtaining white powdery product. The perfluorocarbon solvent is chosen from hydrofluorocarbon (HFC-225), perfluorodimethylcyclobutane, perfluorofuran and perfluoropyran. The additional amount of perfluorocarbon solvent is 60-65 weight% of volume of reaction kettle. The perfluoroperoxide initiator is perfluorobutyl oxide compound. The volume ratio of tetrafluoroethylene with respect to ethylene is (90-10):(10-90);    (3) preparation of suspended polymer of fluororesin, which involves cleaning reaction kettle and drying, vacuuming, supplying nitrogen gas to replace air, controlling the content of oxygen to less than 10 ppm, vacuuming to less than -0.1 MPa, adding pure water containing perfluoropropoxy ammonium carboxylate, and perfluorosulfonyl monomer (I) into the reaction kettle, raising the temperature to 70 degrees C, supplying mixed gas of tetrafluoroethylene and ethylene to allow pressure to reach 4 MPa, continuously adding initiator by metering pump, reacting, continuously supplying mixed monomer of tetrafluoroethylene and ethylene and initiator to set the reaction pressure to 4 MPa, and when the adding amount of mixed gas of tetrafluoroethylene and ethylene reaches 1310 g, the addition of the mixed gas is stopped, and when the pressure of the kettle is reduced to 3.8 MPa, the reaction is stooped, recovering the non-reacted mixed gaseous-phase monomer, discharging the material by discharging valve and pressing to obtain a white polymer wet powder material, washing the wet powder material for several times using deionization water, drying at 100 degrees C, transferring the filtered liquid to the recovering tower to separate and recover the non-reacted ethylene monomer. The additional amount of pure water is 60-65 weight% of the reaction kettle. The additional amount of perfluoropropoxy ammonium carboxylate is 0.2-0.5 vol.% of pure water. The additional amount of perfluorosulfonyl monomer (I) is 60-65 weight% of volume of the reaction kettle. The initiator is potassium persulfate and has concentration of 0.0005-0.0007 mol/L; and    (4) preparation of emulsion polymer of fluororesin, which involves cleaning the reaction kettle and vacuuming, supplying pure nitrogen gas to replace air, controlling the content of oxygen to less than 10 ppm, vacuuming to less than -0.1 MPa, adding pure water containing perfluoropropoxy ammonium carboxylate, and perfluorosulfonyl monomer (I) into the reaction kettle, raising the temperature to 75 degrees C, supplying the mixed gas of tetrafluoroethylene and ethylene to allow pressure to reach 3.8 MPa, continuously adding initiator by metering pump, reacting, continuously supplying the mixed monomer of tetrafluoroethylene and ethylene to set the reaction pressure to 3.8 MPa, and when the adding amount of mixed gas of tetrafluoroethylene and ethylene reaches 1310 g, stopping addition of mixed gas, and when the pressure of the kettle is reduced to 3.6 MPa, stopping reaction, recovering the non-reacted tetrafluoroethylene monomer, discharging the mixture by discharging valve into a crushing barrel, shearing at high speed to demulsify in the crushing barrel to obtain white polymer wet powder material, washing the wet powder material for several times using deionization water, drying at 100 degrees C, transferring the filtered liquid to the recovering tower to separate and recover the non-reacted ether monomer and solvent.