NOVELTY - Preparing hydrocortisone, comprises: ketalation reaction of adding 17 alpha -hydroxy-4-pregnene-3,11,20-trione to glycol and triethyl ortho-formate to give 17 alpha -hydroxy-4-pregnene-11-one-3,20-diethylene ketal (III); reducing (III) to give 11 beta ,17 alpha -dihydroxy-4-pregnene-3,20-diethylene ketal (IV); hydrolyzing (IV); iodination reaction of adding calcium chloride-methanol solution and 11 beta ,17 alpha -dihydroxy-4-pregnene-3,20-dione (V) to trichloromethane; adding acetone to obtained 21-iodine-11 beta ,17 alpha -dihydroxy-4-pregnene-3,20-dione; and hydrolyzing hydrocortisone acetate.    USE - The method is useful for preparing hydrocortisone.    ADVANTAGE - The method provides hydrocortisone with high yield and high purity in an environmentally friendly and simple and economical manner.    DETAILED DESCRIPTION - Preparing hydrocortisone, comprises: (a) ketalation reaction of adding 17 alpha -hydroxy-4-pregnene-3,11,20-trione (II) to glycol and triethyl ortho-formate, adding p-toluenesulfonic acid and stirring the mixture to perform reaction for more than 8 hours at 30-35 degrees C, adding water to analyze and filtrating the mixture, using water to wash until it is neutral, and drying the mixture to obtain 17 alpha -hydroxy-4-pregnene-11-one-3,20-diethylene ketal (III), where the weight ratio of (II) to the glycol to the triethyl ortho-formate to the p-toluenesulfonic acid, is 1:3-50:3-5:0.05-0.1; (b) reduction reaction of adding tetrahydrofuran and methanol to obtained (III), stirring the mixture and adding borohydride, reflowing the mixture for more than 48 hours, reducing the pressure and condensing the mixture to recycle solvent, adding water to analyze and filtrating the mixture, using water to wash until it is neutral, and drying the mixture to obtain 11 beta ,17 alpha -dihydroxy-4-pregnene-3,20-diethylene ketal (IV), where weight the ratio of (III) to the tetrahydrofuran to the methanol to the borohydride, is 1:10-15:1-2:0.3-0.5; (c) hydrolysis reaction of adding alkyl halide and methanol to the obtained (IV), adding inorganic acid to the mixture, stirring the mixture to perform reaction for more than 3 hours at 10-20 degrees C, adding ammonia and water to neutral the mixture, reducing the pressure and condensing the mixture to recycle the solvent, condensing the mixture until there is no alkyl halide, adding water to analyze, filtrating the mixture and using water to wash, and drying the mixture to obtain 11 beta ,17 alpha -dihydroxy-4-pregnene-3,20-dione (V), where the weight ratio of (IV) to the alkyl halide to the methanol to the inorganic acid is 1:4-7:4-7:0.2-0.5; (d) iodination reaction of adding 1/3 of the amount of calcium chloride-methanol solution and (V) to trichloromethane, stirring and dissolving the mixture, then adding calcium chloride and regulating the temperature to 0-5 degrees C, dropping iodine in iodine-calcium chloride-methanol solution prepared using calcium chloride-methanol solution, after adding, carrying out reaction for more than 2 hours at 0-5 degrees C, adding ammonium chloride aqueous solution and filtrating the mixture, extracting water layer from the mixture using trichloromethane and combining them, reducing the pressure and condensing the mixture to recycle solvent, and condensing the condensate until it is dried to obtain 21-iodine-11 beta ,17 alpha -dihydroxy-4-pregnene-3,20-dione (VI), where the weight ratio of the (V) to the trichloromethane to the calcium chloride to the methanol to the calcium oxide to the iodine to the ammonium chloride, is 1:5-10:0.3-0.5: 3-5:0.6-0.8:0.9-1.0:1-1.2; (e) replacement reaction of adding acetone to the obtained (VI), then adding potassium acetate and acetic acid, regulating the temperature to perform reflowing reaction for more than 3 hours, reducing the pressure and condensing the mixture to recycle solvent, adding water to the mixture to analyze, filtrating the mixture using water, and drying the mixture to obtain the hydrocortisone acetate (VII), where the weight ratio of (VI) to the acetone to the potassium acetate to the acetic acid, is 1.4:8-12:0.8-1.2:0.1-0.2; and (f) hydrolysis reaction of adding methanol to the obtained (VII) and adding methanol solution of 2% sodium hydroxide, stirring the mixture at 0-8 degrees C to perform reaction for more than 4 hours, adding acetic acid to neutralize the mixture, reducing the pressure and condensing the mixture to recycle solvent, crystallizing and filtrating the mixture using methanol, and drying the mixture to obtain hydrocortisone, where the weight ratio of (VII) to the methanol to the sodium hydroxide to the acetic acid, is 1:18-25:0.04-0.06:0.06-0.09.