The invention provides a mutant of 7 beta-hydroxyl steroid dehydrogenase, application of the mutant and a synthesis method. The mutant of the 7 beta-hydroxyl steroid dehydrogenase is characterized in that amino acid sequences of the mutant are Seq ID NO:4, and coded nucleotide sequences are Seq ID NO:3; or amino acid sequences of the mutant are Seq ID NO:6, and coded nucleotide sequences are Seq ID NO:5. The mutant, the application and the synthesis method have the advantages that cholic acid compounds, particularly ursodeoxycholic acid, can be catalytically synthesized by the efficient 7 beta-hydroxyl steroid dehydrogenase, mutant enzymes of the 7 beta-hydroxyl steroid dehydrogenase and coenzyme regeneration systems, accordingly, the substrate concentration can reach 100 g/L, the conversion rate is 99.2-99.5%, and the weight yield can reach 94-96%; and the enzymes can be inexpensively and easily obtained by the aid of a fermentation process, accordingly, the production cost and the product quality are superior to the production cost and the product quality of chemical synthesis methods, and the mutant and the synthesis method are applicable to industrial production.