NOVELTY - Preparing ursodeoxycholic acid comprises (i) taking chenodeoxycholic acid, and adding solvent; (ii) adding reagent and catalyst in the solution to obtain 3 alpha -hydroxy modified derivative; (iii) adding the solution with chiral inducer, asymmetric synthesis catalyst and tertiary butanol or n-octyl alcohol, filling hydrogen, irradiating, and converting 7 alpha -hydroxy group into 7 beta -hydroxy group to obtain ursodeoxycholic acid crude product; (iv) washing and filtering; (v) subjecting unreacted chenodeoxycholic acid and other impurities to solvent extraction; and (vi) drying and packaging.    USE - The method is useful for preparing ursodeoxycholic acid (claimed).    ADVANTAGE - The method: is simple; requires less device investment and few chemical reagents; has high conversion rate; fully utilizes the raw materials; increases the production efficiency; reduces production cost and greatly reduces energy consumption; and is suitable for industrial production.    DETAILED DESCRIPTION - Preparing ursodeoxycholic acid comprises (i) taking 1000 g chenodeoxycholic acid as a raw material, and then adding 3-50 l suitable solvent; (ii) adding 5-150 g reagent, and 5-20 g catalyst in the solution to obtain 3 alpha -hydroxy modified derivative; (iii) adding the resulting solution with 0.1-1.2 g chiral inducer, 0.5-5 l asymmetric synthesis catalyst, and 0.2-10 l tertiary butanol or n-octyl alcohol, filling hydrogen under pressure of 0.2-3 MPa, irradiating at 25-80 degrees C under microwave frequency of 300-1000 MHZ and power of 50-1500 W for 1-6 hours, and converting 7 alpha -hydroxy group of chenodeoxycholic acid into 7 beta -hydroxy group to obtain ursodeoxycholic acid crude product; (iv) washing the crude product, and filtering to remove impurities; (v) subjecting unreacted chenodeoxycholic acid and other impurities to solvent extraction; and (vi) drying at 30-80 degrees C under vacuum condition of 10-20 KPa, collecting and packaging to obtain the finished product.