The invention belongs to the technical field of fine chemical engineering, and particularly relates to a purification method of 5-chloro-2-methyl-4-isothiazolin-3-one. The purification method of the 5-chloro-2-methyl-4-isothiazolin-3-one includes the following step that a sample containing the 5-chloro-2-methyl-4-isothiazolin-3-one is eluted in a chromatographic column by using an eluent, whereinthe eluent is a mixed solvent of a first solvent and a second solvent, the first solvent is petroleum ether, and the second solvent is at least one of dichloromethane and ethyl acetate. The purity ofthe 5-chloro-2-methyl-4-isothiazolin-3-one obtained through purification by using the purification method is relatively high, and the highest purity can reach 99.9%.