NOVELTY - Preparing 6 alpha ,9 alpha -difluoroprednisolone-21-acetate-17-butyrate (I) from sterol fermented product comprises e.g. (a) dehydrating 9 alpha -hydroxy-androsta-1,4-diene-3,17-dione (II) and reacting with acetylene to obtain 17 beta -hydroxy-17 alpha -ethynyl-androst-1,4,9 (11)-triene-3-one (IV), then dehydrating, carbonylating and epoxidizing, oxidizing and deprotecting of (IV) to obtain 21-hydroxy-16 alpha ,17 alpha -epoxy-pregn-1,4,9(11)-triene-3,20-dione (IX), and ring opening and debromination of (IX) to obtain 17 alpha ,21-dihydroxy-pregn-1,4,9(11)-triene-3,20-dione (XI).    USE - The method is useful for preparing 6 alpha ,9 alpha -difluoroprednisolone 21-acetate 17-butyrate (claimed).    ADVANTAGE - The method: avoids generated heavy metal pollutants (chromium) during traditional industrial production corticosteroid; and is environmentally friendly.    DETAILED DESCRIPTION - Preparing 6 alpha ,9 alpha -difluoroprednisolone-21-acetate-17-butyrate of formula (I) from sterol fermented product comprises (a) dehydration of 9 alpha -hydroxy-androsta-1,4-diene-3,17-dione of formula (II) to obtain androsta-1,4,9(11)-triene-3,17-dione of formula (III), (b) reacting (III) with acetylene at 17-postion to obtain 17 beta -hydroxy-17 alpha -ethynyl-androst-1,4,9 (11)-triene-3-one of formula (IV), (c) dehydration of hydroxy group of (IV) at 17-position to obtain 17-ethynyl-androst-1,4,9 (11),16(17)-tetraene-3-one of formula (V), (d) carbonylation of (V) at 20 position to obtain pregn-1,4,9(11),16(17)-tetraene-3,20-dione of formula (VI), (e) epoxidation of (VI) at 7th position to obtain 16 alpha ,17 alpha -epoxy-pregn-1,4,9(11)- triene-3,20-dione of formula (VII), (f) oxidizing (VIII) with periodate compound (introducing hydroxy at 21 position) to obtain 21-hydroxy-3-oxo-20,20-dimethoxy-16 alpha ,17 alpha -epoxy-pregn-1,4,9(11)-triene of formula (VIII), (g) deprotecting of (VIII) to obtain 21-hydroxy-16 alpha ,17 alpha -epoxy-pregn-1,4,9(11)-triene-3,20-dione of formula (IX), (h) ring opening of (IX) using hydrogen bromide to obtain 16 beta -bromo 17 alpha , 21-dihydroxy-pregn-1,4,9(11)-triene-3,20-dione of formula (X), (i) removal of bromine in (X) at beta position to obtain 17 alpha ,21-dihydroxy-pregn-1,4,9(11)-triene-3,20-dione compounds of formula (XI), (j) reacting 17 alpha ,2-dihydroxy group of (XI) with acid ester to obtain pregn-1,4,9(11)-triene-3,20-dione-17 alpha -butyrate compounds of formula (XII), (k) ring opening of (XII) by acid catalysis to obtain 21-hydroxy-pregn-1,4,9(11)-triene-3,20-dione-17 alpha -yl butyrate of formula (XIII), (i) acetylating hydroxy group of (XIII) at 21 position to obtain 21-acetyloxy-pregn-1,4,9 (11)-triene-3,20-dione-17 alpha -yl-butyrate of formula (XIV), (m) beta-epoxidation of (XIV) at 9,11 double bond to obtain 21-acetyloxy-9 beta ,11 beta -epoxy-pregn-1,4-diene-3,20-dione-17 alpha -yl butyrate of formula (XV), (n) enolization and esterification of (XV) to obtain 3 alpha ,21-diacetoxy-20-oxo-9 beta ,11 beta -epoxy-pregn-1,3,5-triene-17 alpha -yl-butyrate of formula (XVI), (o) electrophilic fluorinating (XVI) at 6th position to obtain 21-acetyloxy-6 beta -fluoro-9 beta ,11 beta -epoxy-pregn-1,4-diene-3,20-dione-17 alpha -yl-butyrate of formula (XVII) and 21-acetyloxy-6 alpha - fluoro-9 beta ,11 beta -epoxy-pregn-1,4-diene-3,20-dione-17 alpha -yl-butyrate of formula (XVIII), and (p) nucleophilic fluorination and ring opening of (XVII) and (XVIII) to obtain (I).