The disclosure provides a catalyst system and a method for ethylene oligomerization using this. The catalyst system contains: ligand a, containing carbene groups of imidazole ring type; transition metal compound b, that is one of IVB˜VIII group metal compounds; activator c, that is a compound containing III A group metals; the ligand a contains at least one group as shown in general formula I: in which, bridging group A contains a main chain including alkyl, alkenyl, aryl groups or the combination of them and the first heteratom; E is a linear or cyclic group containing the second heteroatom; R is a hydrocarbyl group. The catalyst system is especially used for trimerization and tetramerization of ethylene. The catalyst system has high selectivity for 1-hexene and 1-octene, low selectivity for 1-butene and 1-C10+, and the total percent content of C6˜C8 linear □α-olefin in the product is more than 90% by mass.