The invention discloses an ethylene oligomerization catalyst composition which comprises a ligand compound shown in formula I, a transition metal compound, an aluminum-containing promoter, tert-butyl hydroperoxide and an organic solvent, and in the formula I, X is O or S; R1, R2, R3 and R4 are identical or different and are independently selected from hydrogen, alkyl, alcoxyl and halogen. The invention further provides an ethylene oligomerization method. The ethylene oligomerization method comprises the following step: in the presence of the ethylene oligomerization catalyst composition, performing ethylene oligomerization reaction. The ethylene oligomerization catalyst composition disclosed by the invention has the advantages of high activity and high selectivity.