The invention discloses an O,O-diethyl chlorothiophosphate and a preparation method thereof, belonging to the technical field of insecticides. The preparation method comprises the following steps: mixing and stirring phosphorus pentasulfide and O,O-diethyl dithiophosphate, dripping a mixture of absolute ethyl alcohol and a catalyst A, preserving heat for 10 to 50min, and then filtering to obtain O,O-diethyl dithiophosphate, wherein the catalyst A is prepared from at least one of triethylamine, pyridine, ethylenediamine and carbonate; stirring the O,O-diethyl dithiophosphate at 30 to 50 DEG C, and introducing chlorine to obtain an intermediate product; heating the intermediate product to 80 to 100 DEG C, adding water, performing stirring reaction for 1 to 3h, then performing suction filtration, and enabling filtrate to stand, thus layering to obtain an oil phase and a water phase; dehydrating the oil phase, then evaporating, and collecting a fraction. The preparation method is capable of avoiding side reaction and reducing a reaction residue amount. In addition, the invention provides the O,O-diethyl chlorothiophosphate prepared by using the method.