Olefin polymerization catalyst as well as preparation method and application thereof

The invention discloses a catalyst for olefin polymerization. The catalyst comprises the following components or a reaction product of the following components: (1) a magnesium compound; (2) a nickel compound; (3) a nitrogen-oxygen heterocyclic compound; (4) an organoaluminum compound; (5) an electron donor compound; and (6) an inorganic oxide carrier, and the nickel compound is selected from compounds represented by formula (I). The catalyst provided by the invention is simple and convenient to load and form, convenient for industrial production, good in product grain shape and capable of spontaneously inducing branching, and a resin product with medium and low density can be obtained under the condition that a comonomer is not added. Besides, the catalytic system can obtain a resin product with low melt index and low density under the normal production condition of the existing gas-phase polymerization process, keeps the advantages of high catalytic activity and high stacking density, and particularly has s