The invention discloses an ethylene trimerization catalyst composition which comprises diphosphine ligand shown in formula I, a transition metal compound, an aluminum-containing promoter and peroxide, and in formula I shown in the specification, R1 and R4 are identical or different and are independently selected from alkyl, a naphthenic base and an aromatic group; R' and R'' are hydrogen or are independently selected from alkyl, cycloalkane and aromatic groups. Meanwhile, the invention further provides a method for ethylene trimerization reaction with the catalyst composition. The ethylene trimerization catalyst composition is high in catalysis activity on the contrary and good in catalysis activity and selectivity of 1-octylene.