A Bradyrhizobium monooxygenase, a gene for encoding the monooxygenase, a recombinant expression vector comprising the gene and a recombinant transformant, a method of preparing the monooxygenase by the recombinant expression transformant, and a method of preparing an optically pure chiral sulfoxide by the monooxygenase, in particular to a method of preparing prazole drugs by means of catalyzing the asymmetric oxidation of thioether, a prazole precursor. As compared with other methods of preparing an optically pure sulfoxide, the product produced by the monooxygenase of the present invention as a catalyst has high optical purity, avoids the generation of the byproduct sulfone, and has advantages of mild reaction conditions, simple and convenient operations, easy amplification, etc.