The invention relates to a preparation method of an ethylene-tetrafluoroethene copolymer. The main improvement of the invention is that a polymerization reaction is carried out under the action of a redox mixing initiator by using a mixes solution of water and an organic solvent as a reaction medium in a process of preparation of the ethylene-tetrafluoroethene copolymer by an emulsion polymerization method using ethylene and tetrafluoroethene as raw materials, wherein the polymerization temperature is 40-55 DEG C, and sintering the obtained polymer at 320-350 DEG C after the polymerization iscompleted. The ETFE resin prepared by the method of the invention has good thermal stability during processing and application and has a melting point of 240-275 DEG C, tensile strength of amount 40-52 MPa and elongation at break of 300%. Moreover, the method of the invention is environmentally friendly, benefits for the industrialization and can realize mass production.