The invention discloses a preparation method for a mixture of an ethylene-undecylenyl alcohol copolymer and ultrahigh molecular weight polyethylene. The preparation method has the advantages that a co-catalyst of polar monomers and a catalyst is wrapped with a polyethylene microsphere granule, and during polymerization, the composition of a solvent is changed to regulate a solubility parameter of the solvent to further control the swelling rate of polyethylene to control the releasing rate of the co-catalyst of the polar monomers and the catalyst, so that the problem of influence of the polar monomers on the polymerization activity of the ethylene homopolymerization catalyst and a chain transfer process is solved, the polymerization activity of the ethylene homopolymerization catalyst and the molecular weight (more than 1,500,000g/mol) can be effectively ensured, and the mixture of ultrahigh molecular weight polyethylene and the ethylene-undecylenyl alcohol copolymer can be prepared by the in-situ polymerization of ethylene in a single reactor.