The present invention relates to preparation and application of an olefin polymerization catalyst. The main catalyst is composed of a carrier, a transition metal halide, and an organic alcohol compound. The transition metal halide is obtained by in-situ reaction between titanate and silicon halide. The molar ratio of the carrier to the transition metal halide to the organic alcohol compound is 1 to 0.01-20 to 0.1-6. The molar ratio of the titanate to the silicon halide is 1 to 0.5-2. The co-catalyst is an organo-aluminum compound. The molar ratio of the transition metal halide in the main catalyst to the co-catalyst is 1 to 30-500. Particles of the catalyst are good in morphology and are in a spherical shape. The catalyst has high activity and the polymer obtained by using the catalyst has a high molecular weight. The catalyst is applicable to a slurry method, a vapor phase polymerization process, or a combined polymerization process. The preparation method thereof is simple, has low requirements on devices, and produces little pollution on the environment.