Ethylene selective trimerization/tetramerization catalyst system as well as preparation method and application thereof

The invention provides an ethylene trimerization and tetramerization catalyst system as well as a preparation method and application thereof, and belongs to the field of olefin selective trimerization or tetramerization catalysis. The catalyst system is characterized by comprising: a ligand a; a transition metal compound b; the transition metal compound b is a compound containing IIIB-VIIB group or VIII group transition metal; an activator c; the activating agent c is an organic compound containing IIIA group elements; the ligand a at least contains one group as shown in a formula (I), and R1, R2, R3, R4, R5, R6 and R7 can be the same or different and are respectively and independently selected from alkyl and derivatives thereof, aryl and derivatives thereof, halogen and heteroatom-containing substituents. The catalyst system provided by the invention has the advantages of high catalytic activity, high selectivity of target products 1-hexene and 1-octene, less 1-butene and 1-C10 < + >, less by-product oligome