The invention provides a method for preparing an
ethylene polymer. The method comprises a step that
ethylene and alpha-olefin polymerize in the presence of H2, an
inert gas and a supported
metallocene catalyst in a single reactor, wherein the
polyethylene density is 0.925-0.950g / cm<3>, the
polyethylene fusion index is 0.1-10g / 10min, the molecular
weight distribution of
polyethylene is 2-10, the
molar ratio of alpha-olefin to
ethylene is 0.05-0.2, the supported catalyst is composed of a
metallocene compound supported on a porous supporter and a cocatalyst, the
metallocene compound is a bridged or non-bridged organic
metal compound and contains one or more eta-ligands, the
metal is in Subgroup 4, 5 or 6 in
Periodic Table of the Elements, the cocatalyst is
methylaluminoxane or modified
methylaluminoxane, and the supporter can be an
inorganic oxide, an
organic oxide, a
polymer or a mixture. Medium-density polyethylene can be obtained through utilizing the method provided by the invention, and polyethylene has good mechanical performances, can be used for
extrusion molding or injection molding, and can be especially used in the hot water tube field.