Abstract: The vacuum volatilization method was used to remove the free negative divalent sulfur in the sulfidation solution of molybdate. The effects of ammonium-sulfur ratio, volatilization time, temperature, tungsten and molybdenum concentration on the sulfur volatilization and the molybdenum removal during the subsequent quaternary ammonium salt extraction were investigated. The results show that the volatilization temperature and ammonium-sulfur ratio are important factors affecting the sulfur volatilization rate. Under the condition of -80 kPa, volatilization temperature 60 ℃ and ammonium-sulfur mole ratio greater than 3, a better sulfur volatilization rate can be obtained. Furthermore, the hydrogen ions produced by ammonium decomposition in the process of vacuum volatilization facilitate the sulfidation reaction of molybdate, which is beneficial to the separation of tungsten and molybdenum. After fully sulfidation of molybdate solution, the vacuum volatilization operation can not only recover some sulfur, but also obtain better molybdenum removal effect in the subsequent quaternary ammonium salt extraction process. However, some tungsten-containing crystals will be formed when the ammonium concentration in the solution is too high and the vacuum volatilization is carried out to a certain extent.