The invention relates to a catalyst, a preparation method thereof, a composition prepared from the catalyst and applications of the catalyst and the composition. The catalyst has a structural formulashown in the description. The preparation method of the catalyst comprises the following steps: firstly, dissolving 2-hydroxy-1,4-naphthoquinone and a substituted aniline into an organic solvent, andperforming a reaction to obtain an aniline naphthoquinone ligand; secondly, dissolving the aniline naphthoquinone ligand into an organic solvent, adding a hydrogen removing reagent, and performing a reaction to obtain a ligand salt compound; and finally, adding a nickel precursor, and performing a reaction to obtain the catalyst. The catalyst provided by the invention can form the composition witha cocatalyst, and the catalyst and the composition can be used to catalyze polymerization of olefin monomers and copolymerization of ethylene and a polar monomer. The catalyst provided by the invention can effectively regulate and control the molecular weight and chain structure of an olefin polymerization product by using different cocatalysts for cooperation, the catalyst has high catalytic activity, can achieve certain activity without the need of the cocatalysts, and has good economical performance, and the composition prepared from the catalyst also has higher catalytic activity.