The present invention discloses a preparation method of 7-dehydrocholesterol. The saponified waste from the prior production of vitamin D3 is always treated as waste liquid, which wastes the resources greatly and causes the environmental pollution. The 3-carbonyl holest-4, 6-diene is prepared by the oxidation reaction of the 3-hydroxyl cholest-4, 6-diene vitamin D3 and weak oxidants under the nonpolar or weak polar solvent; then the mixture of 3-carbonyl holest-4, 6-diene and 3-hydroxyl cholest-5, 7-diene is prepared by the reduction reaction of the 3-carbonyl holest-4, 6-diene under weak polar solvent; the crystal of 3-hydroxyl cholest-5, 7-diene and the filtrate rich in 3-hydroxyl cholest-4, 6-diene is obtained after recrystallization purification of the mixture, and the filtrate can be used as materials of oxidation reaction after eliminating the solvent. The present invention prepares the 7-dehydrocholesterol with saponified waste, thereby the saponified waste is used efficiently and the environmental pollution is reduced.