A catalyst comprises a heteroatom-containing ligand a, a transition metal compound b and an activator c. The heteroatom-containing ligand a is a compound in conformity with the general formula I, or a compound formed by connection of more than two structure units in conformity with the formula I through groups or chemical bonds. The transition metal compound b is a compound containing IVB-VIII family metal. The activator c is a compound containing an IIIA family element. The catalyst can be used for catalyzing ethylene oligomerization.