The invention provides a preparation method of a polypropylene main catalyst. The preparation method comprises dissolving magnesium halide in a solvent system of an organic epoxide and an organophosphorus compound to obtain a uniform solution, adding a halogenated compound of transition metal titanium or its derivative into the solution in the presence of a succinic anhydride compound precipitation aid so that titanium/magnesium-containing solid precipitates are separated, loading the solid precipitates with at least one surface modifier, at least one transition metal titanium halide or its derivative, and an internal electron donor compound, and washing the precipitates through a diluent so that a main catalyst is obtained. Through the succinic anhydride compound with a specific structure as a precipitate aid, the main catalyst with excellent comprehensive properties is obtained. In propylene polymerization, the main catalyst has high polymerization activity and stereoselectivity, improves polymer molecular weight distribution, has less fine powder content, is convenient for industrial processing application and is conducive to development of polymers with different types.