N-(n-butyl)thiophosphoric triamide (NBPT) was synthesized successfully by modified “one-pot” method. During the experiments, the ammonia was used as acid-binding agent to replace traditional liquid reagent and the raw materials of subsequent amination reaction. The effect of the reaction temperature, the mole ratio of substrates, the flow rate of ammonia, the intermittence time of flowing ammonia and the temperature of amination reaction on yield of the product were investigated. The optimized experimental parameters were as follows: the reaction temperature was 0 ℃, the mole ratio of n-butylamine (0.42mol) and thiophosphoryl chloride (0.35mol) was 1.2:1, ammonia flow rate was 15 mL/min, the intermittence time of flowing ammonia was 30 min and the temperature of amination reaction was 10 ℃. Under the optimized conditions, the yield of NBPT was 75.6% and the purity was 97%. The sample with the highest yield was characterized by infrared spectroscopy (IR), elemental analysis, nuclear magnetic resonance spectrum (NMR) and liquid chromatograph-mass spectrometer (LC-MS). The synthesis process, an innovative green one, had advantages of simple operation steps and low production cost. At the same time, the only by-product was ammonium chloride.