The present invention discloses an ethylene tetrapolymerization catalyst composition, which comprises a diphosphine ligand represented by a formula I, a transition metal compound, an aluminum-containing co-catalyst, water and an organic solvent, wherein R' and R" are different and are independently selected from hydrogen and tert-butyl, and the existing 4 R can be the same or different and are respectively and independently selected from alkyl, cycloalkyl and aryl. The invention further discloses a method for carrying out an ethylene oligomerization reaction by using the catalyst composition. The formula I is defined in the specification.