The invention discloses a preparation method of a 25-hydroxyvitamin D3 intermediate, which comprises the following steps: taking 3 beta-tert-butyldimethylsiloxy ergosta-5, 7, 22-triene as a
raw material, carrying out DA reaction on the 3 beta-tert-butyldimethylsiloxy ergosta-5, 7, 22-triene and phthalylhydrazine under the action of
lead dioxide and
acetic acid to generate 3 beta-tert-butyldimethylsiloxy-5a, 8a-(1, 4-dioxo-1, 2, 3, 4-tetrahydro-
phthalazine-2, 3-diyl) ergosta-6, 7, 22-triene, and reacting the 3 beta-tert-butyldimethylsiloxy-5a, 8a-(1, 4-dioxo-1, 2, 3, 4-tetrahydro-
phthalazine-2, 3-diyl) ergosta-6 and carrying out simple
crystallization to obtain a high-content
crystal product. Reaction raw materials are simple and easy to obtain,
silicon ether bond breakage is avoided, meanwhile, raw materials with high dangerousness are not used, no other toxic and harmful by-product residues exist in a product, safety is high in the production process, and the product meets the use standard. Therefore, the whole preparation method needs less manpower, material and financial resources, is high in safety and is suitable for large-scale industrial production.