The invention belongs to the technical field of catalysts and discloses a vinyl polymerization catalyst and its preparation method. The vinyl polymerization catalyst comprises an alkoxymagnesium compound, a chlorosilane compound, a titanium compound and a chlorinated organic aluminum compound. The preparation method comprises that the alkoxymagnesium compound and the chlorosilane compound undergo a reaction to produce a suspension liquid, the suspension liquid and a titanium compound further undergo a reaction and the suspension liquid formed in the step 2 contacts with the chlorinated organic aluminum compound and undergoes a reaction to produce a catalyst suspension liquid. In polymer resin preparation, the catalyst reduces oligomer content, has high catalysis activity and good particle forms and is suitable for ethene polymerization double-reactor polymerization.