The invention discloses a preparation method of an ethylene/alpha-olefin copolymer, and application of the ethylene/alpha-olefin copolymer in pipeline transmission, belonging to the technical fields of synthesis and preparation of macromolecular compounds as well as pipeline transmission systems. The concrete preparation method adopts a mass polymerization method, and comprises the steps of mixing C6-C10 alpha-olefin, C12-C14 alpha-olefin and a C16-C18 alpha-olefin and ethylene according to a certain proportion, and taking the mixture as a raw material; adopting a Ziegler-Natta catalyst system as a catalyst, adding a plasticizing agent, a viscosity reducer, a molecular weight regulator, and the like at the same time, introducing nitrogen, initiating a reaction by means of heating and ultraviolet radiation, and synthesizing the ethylene/alpha-olefin copolymer; the product can be used as an oil drag reducer so as to be applied to pipeline transmission of crude oil. The olefin copolymer macromolecule oil drag reducer prepared by the method is higher in conversion rate, low in gelling rate, low in preparation cost, not easy in implosion and low in molecular weight dispersity, can be very easily dispersed in high wax content oil, can greatly improve the pipeline transmission efficiency of the high wax content oil, reduces pipe wall wax deposition effect, and reduces the accidents of pipeline wax blockage.