The invention relates to a preparation method of fluorinated ethylene-propylene (FEP) resin with good melting uniformity in an extruding processing process. The preparation method takes hexafluoropropylene (HFP) and tetrafluoroethylene as raw materials and the mol percent of the hexafluoropropylene in a gas-phase environment of a reaction kettle is controlled to be 50 to 75 percent in a process ofpreparing the fluorinated ethylene-propylene resin through an emulsion polymerization method. According to the method provided by the invention, the content of the hexafluoropropylene (HFP) in a gasphase of the kettle is controlled to be 50 to 75 percent in a polymerization process, the melting point of the obtained FEP resin can be stabilized at 260 to 270 DEG C, the tensile strength is about 30 MPa and the elongation at break is greater than 350 percent; a phenomenon that partial melting is incomplete in a material heat melting and processing process, caused by the fact that the molecularweight distribution of the FEP in the polymerization process is not uniform or the distribution of the HFP on a molecular chain segment is not uniform is effectively solved.