The invention discloses a synthesis method of captan or folpet, which comprises the following steps: uniformly mixing 1, 2, 3, 6-tetrahydrophthalimide or phthalimide, perchloromethyl mercaptan, an organic solvent and a catalyst to obtain a mixture a; or uniformly mixing 1, 2, 3, 6-tetrahydrophthalimide or phthalimide, an organic solvent and a catalyst to obtain a mixture b; dropwise adding an inorganic alkali aqueous solution into the obtained mixture a, and after dropwise adding, carrying out heat preservation for reaction to synthesize captan or folpet; or respectively dropwise adding perchloromethyl mercaptan and an aqueous solution of inorganic alkali into the obtained mixture b, and carrying out heat preservation and reaction after dropwise adding is completed, so as to synthesize captan or folpet. The organic solvent, the catalyst and the water are introduced at the same time, the operation mode is optimized, excessive contact of the raw materials, the product and alkali is reduced, decomposition of the raw materials and the product is reduced, the product yield is increased, the wastewater amount is greatly reduced, the environmental protection requirement is greatly reduced, and the environmental friendliness is greatly improved.