The invention discloses a preparation method of polyethylene. The preparation method has the advantages that by an in-situ free radical polymerization method of styrene in a porous carrier, the styrene, polymerization monomers and initiator are diffused into the carrier before the polymerization is initiated; the porous carrier with the ducts being evenly filled with styryl polymer can be obtained, the catalyst is loaded further, then heating is performed to allow the styryl polymer in the ducts to crosslink, and the polyethylene product with high catalyst activity, long half-life period and product bulk density larger than 320g/cm<3> is obtained through the mass transfer resistance control of the styryl polymer to vinyl monomers, the high mechanical strength of the porous organic carrier and the swelling behaviors of the styryl polymer in the limited ducts.