The present invention relates to an ethylene oligomerization method including ethylene oligomerization in the presence of a main catalyst shown as a formula (I), a co-catalyst, an organic solvent and water, wherein four R groups may be same or different, are each independently selected from alkyl, cycloalkyl and aryl groups, and preferably phenyl or substituted phenyl groups; R' and R '' are different, and are each independently selected from hydrogen and t-butyl group, M is a transition metal, X is selected from halogens, and n is an integer of 1 to 3.