Abstract: Identification of active fraction in petroleum sulfonates has great guidance significance to the choice of crude oil and interfacial quality of product, and is be valuable for the production control of petroleum sulfonate products with improved quality, once the key active fraction has been traced and recognized accurately. Active fraction with excellent interfacial activity was successfully separated and obtained from the petroleum sulfonate samples used in Changqing oilfield by the preparative liquid chromatography combined mass spectrum analysis and interfacial tension determination. The results showed that the mass percentage of active fraction was 7.3%, and it could produce ultralow interfacial tension (< 1.0×10^-3mN/m) against different oil phases, including n-pentane to n-hexadecane, and crude oils from different oilfields. Besides, the active fraction was mainly mono-sulfonate, and the average molecular weight and molecular weight distribution range were 414 (not including Na⁺) and 380~450, respectively. The main components were a mixture of heptadecyl benzene sulfonate and octadecyl benzene sulfonate, containing a variety of isomers.