This invention provides an amidasemutant and its application in the preparation of ( S The application of pyrimethanil belongs to the fields of genetic engineering and enzymecatalysis technology. Compared with the amidase shown in SEQ ID No. 1, the amidasemutant of this invention contains one or more mutations at the following sites: alanine at position 132 is mutated to leucine; serine at position 205 is mutated to glycine; alanine at position 211 is mutated to glycine. Compared with wild-type amidase, the enzyme activity and stereoselectivity of the amidase mutant of this invention are significantly improved; based on the mild and simple preparation method of this mutant, the obtained ( S )-Pyrimethanil possesses excellent chiral selectivity and product optical purity, which aligns with the development trend of green pesticide creation and has significant industrial application value.