In view of the poor accuracy and long time in determining the content of sulfur monochloride and sulfur dichloride in thionyl chloride, the isoabsorptive point dual-wavelength spectrophotometry was researched. The results show that the total detection rate of sulfur monochloride and sulfur dichloride in thionyl chloride is 98% ~ 100% by the method. The method is simple, rapid and accurate, and suitable for the quality control and detection of thionyl chloride.