The invention relates to a preparation method of an important intermediate of cortisone acetate, and in particular relates to a preparation method of cortisone acetate intermediate 17alpha-hydroxypregnen-4-alkene-3,11,20-triketone. The preparation method comprises the following steps of (1) preparing an oxidization reagent A; (2) oxidizing 11alpha, 17 alpha-dihydroxypregnen-4-alkene-3,20-diketone to generate 17alpha-hydroxypregnen-4-alkene-3,11,20-triketone. According to the invention, the problem that industrialized production of 17alpha-hydroxypregnen-4-alkene-3,11,20-triketone in a proper workshop is carried out is solved, so the preparation method disclosed by the invention has the characteristics of lower cost and proper yield, and is more suitable for industrialized large-scale production; the reference quality yield of the 17alpha-hydroxypregnen-4-alkene-3,11,20-triketone obtained by the preparation method is 90-100%, and the reference high performance liquid chromatography purity of the 17alpha-hydroxypregnen-4-alkene-3,11,20-triketone is 95-99.9%.