Abstract: The content of active substance in sodium petroleum sulfonate was determined by synchronous back-titration in the presence of the anionic surfactant,CTMAB. Using sodium lauryl sulfate solution as titrant and bromophenol blue as indicator. Based on the principle of synchronous back-titration,the formula for calculation of active substance content was proposed. The accuracy of the proposed method was tested by analyzing 5 samples with known active substance contents prepared by mixing 2 samples of sodium petroleum sulfonate (with known active substance contents of 45.30% in sample 1# and 8.49% in sample 2#) in different ratios. Relative errors between the determined values and the calculated known values were found to be less than ±2.0%. Precision was tested on the 2 standard samples mentioned above,values of RSD′s (n=10) found were 0.9% for sample 1# and 2.0% for sample 2#.