The invention discloses a synthetic method of chlorpyrifos, which comprises the following steps of: enabling trichloro-acetic chloride, acrylonitrile, o-dichlorobenzene and a CuCl catalyst to react to obtain a crude product; adding the crude product into a reaction kettle containing water, a catalyst and alkali, raising the temperature and backwardly flowing for 3-4 hours, standing and separating out a solvent, cooling and crystallizing a water phase, centrifuging to obtain a trichloropyridine alcohol potassium tide product and drying to obtain a trichloropyridine alcohol potassium finished product; and adding the trichloropyridine alcohol potassium finished product into the reaction kettle, adding water, boric acid and a catalyst, agitating, raising the temperature to 75-80 DEG C, maintaining the pH to be equal to 9-10, dripping ethyl chloride, maintaining for 3-5 hours, standing and phase separation, separating out an oil phase, adding the oil phase into absolute ethyl alcohol for recrystallization, filtering and air-drying to obtain a chlorpyrifos finished product. The synthetic method has the advantages of easiness in operation, high yield and reduced environment pollution and provides guarantee for sustainable development.