A method for producing an ethylene polymer which comprises continuously polymerizing ethylene alone or copolymerizing ethylene and a C.sub.3-C.sub.8 .alpha.-olefin by multistage polymerization in a plurality of polymerization reactors connected in series in the presence of a chromium catalyst having at least a part of chromium atoms converted to hexavalent chromium carried on an inorganic oxide carrier by calcination-activating in a non-reducing atmosphere, wherein an organoaluminum compound of the formula (1): (R.sup.1)xAl(OR.sup.2)y(OSiR.sup.3R.sup.4R.sup.5)z (1) (in the above formula, R.sup.1 and R.sup.2 may be the same or different, each represents a C.sub.1-C.sub.18 alkyl group or a C.sub.6-C.sub.18 aryl group, R.sup.3, R.sup.4 and R.sup.5 may be the same or different, each represents a C.sub.1-C.sub.18 alkyl group or a C.sub.6-C.sub.18 aryl group, R.sup.1, OR.sup.2 and OSiR.sup.3R.sup.4R.sup.5 respectively may be the same or different when each has a plurality of kinds, and x, y and z are respectively 0.ltoreq.x.ltoreq.3, 0.ltoreq.y<3, 0.ltoreq.z<3, and x+y+z=3) is introduced at least one or all of the polymerization reactors.