Preparation method of linear low density polyethylene

The invention discloses a preparation method of linear low density polyethylene. By using an alkylaluminium-preactivated supported TiCl4/MgCl2 catalyst as a main catalyst, using alkylaluminium as a cocatalyst and using H2 as a molecular weight modifier, ethene and alpha-olefin undergo a slurry polymerization reaction so as to obtain the linear low density polyethylene. The preparation method of the linear low density polyethylene is suitable for a production device of linear low density polyethylene with the supported TiCl4/MgCl2 catalyst used as a main catalyst. In comparison with a metallocene catalysis system, the production process of the invention is easy to control, use amount of the cocatalyst is low, production cost is low, insert quantity of alpha-olefin in the prepared linear low density polyethylene is high, composition and distribution of a copolymer are uniform, tensile property of the copolymer is good, and the copolymer has better application characteristics.