The improved procedure for preparing N-n-butylthiophosphoric triamide was described. The important intermediate N-n-butylaminothiophosphoryl dichloride was synthesized from thiophosphoryl chloride reacted with n-butylamine in the tubular reactor, followed by ammonolysis by NH3 for about 3h to give the title compound. The overall yield was 65.1%. The structure of the product was characterized by IR、ESI - MS、1H - NMR and 13C - NMR. In the meanwhile, a few factors including molar ratio of raw materials, reaction temperature and residence time are investigated. And the optimum condition were: Molar ratio of thiophosphoryl chloride to n-butylamine is 1 : 1.5, residence time is 30s, tube length 14m, tube diameter 3mm, 28 ~ 30℃。