Methods for the preparation of N-substituted isothiazolinone derivatives are provided, which include reacting the N-substituted 3-mercaptopropionamide of formula II or N,N'-disubstituted 3,3'-dithiadipropionamide with sulfurylchloride, in the absence of solvents. Methods for the preparation of compounds represented of formula III are also provided, which include reacting the methyl ester of formula IV with amine of formula V in the presence of methanol as solvent. The present methods need not use any other solvent, and therefore can lower the cost and reduce the pollution.