The invention discloses a catalyst composition for ethylene tetramerization, which comprises a diphosphine ligand shown as formula I as shown in the specification, a transition metal compound, an aluminiferous promoter and tert-butyl hydroperoxide, wherein R1-R4 are the same or different and are independently selected from substituted or unsubstituted alkyl, cycloalkyl and aryl; and R' is substituted or unsubstituted alkyl, cycloalkyl or aryl. The invention further discloses a method of ethylene tetramerization by using the composition. The catalyst composition can improve ethylene tetramerization activity and selectivity of 1-octylene.