NOVELTY - Preparing 25-hydroxy cholesterol involves protecting compound 3-hydroxy (II); in solvent I, catalyzed by 4-dimethyl amino pyridine and in the presence of acid-binding agent, reacting 24-dehydrocholesterol and carboxylic acid anhydride or acyl chloride at 10 degrees C under reflux condition for 10-24 hours to obtain acylated 24-cholesterol (II); reacting the acylated 24-cholesterol (II) in solvent II, deionized water and a halogenating agent at the temperature of -20 to -1 degrees C for 2-4 hours to obtain 25-hydroxy cholesterol.    USE - For preparing 25-hydroxy cholesterol (claimed).    ADVANTAGE - The method has simple process, less side effects, high yield characteristics, and is environmentally friendly.    DETAILED DESCRIPTION - Preparing 25-hydroxy cholesterol involves protecting compound 3-hydroxy of formula II; in solvent I, catalyzed by 4-dimethyl amino pyridine and in the presence of acid-binding agent, reacting 24-dehydrocholesterol and carboxylic acid anhydride or acyl chloride at 10 degrees C under reflux condition for 10-24 hours, the molar ratio of 24-dehydrocholesterol and carboxylic acid anhydride or acyl chloride is 1:1-2:2, where the weight ratio of 4-dimethyl amino pyridine to 24-dehydrocholesterol is 0.005-0.05:1 the molar ratio of the acid-binding agent to 24-dehydrocholesterol is 1.8-10:1, subjecting the obtained reaction liquid to post-treatment to obtain acylated 24-cholesterol (II); reacting the acylated 24-cholesterol in solvent II, deionized water and a halogenating agent at the temperature of -20 to -1 degrees C for 2-4 hours, where the molar ratio of the acylated 24-cholesterol to the halogenating reagent is 1:1-1.5, each 1 mmol of acylated 24-cholesterol is matched with 8-12 mL of deionized water, the obtained reaction liquid is diluted and then extracted, extraction liquid is washed and dried, then subjecting the obtained dried product to column chromatography separation to obtain hydroxyl halogen product; subjecting the product to dehalogenation reaction, in a solvent III, the hydroxyl halide reacts with a hydrogenating agent at the temperature of 20-70 degrees C; for 3-8 hours, the molar ratio of the hydrogenating agent and hydroxyl aluminum halide product is 2-6:1, the obtained reaction liquid is cooled to room temperature and then excessive aluminum hydrogenating agent in the reaction liquid is quenched by water, and then washed and extracted with acid; the extraction liquid is washed and dried to obtain 25-hydroxy cholesterol crude product, recrystallizing the 25-cholesterol crude product to obtain 25-hydroxy cholesterol (I).