The invention discloses a preparation method of O, O '-dimethyl thiophosphoric amide. The method comprises the following steps: adding water and O, O '-dimethyl thiophosphoryl chloride into a reactioncontainer, stirring, introducing liquid ammonia at a set temperature to enable the mass ratio of the water in the reaction container to the introduced liquid ammonia to be 1: (0.30-0.35) for ammoniation reaction, and stopping introducing the liquid ammonia when the pH value of reaction liquid reaches 8-9; after the reaction is finished, carrying out liquid-solid separation, separating out a water-phase liquid, an organic-phase liquid and a white ammonium chloride solid, and introducing the separated water-phase liquid into a reaction container for cyclic utilization; and distilling the organic phase liquid to recover the solvent, thereby obtaining O, O '-dimethyl thiophosphoric amide. According to the method, the ammonia ratio of a reaction system is increased, the speed is high, the reaction is complete, no wastewater is discharged, the yield is high and the quality is good.