In order to reveal the structural cause of the interfacial activity of petroleum carboxylate and further research the combination law of petroleum carboxylate with sulfonate, three fractions of Daqing crude oil with average carbon number of 23.16, 26.01, 28.70, respectively, were selected as the raw materials to prepare a kind of petroleum carboxylate whose average carbon number is close to the carbon number of sodium oleate. The interfacial activities of sodium oleate and the petroleum carboxylate were detected and compared. Meanwhile, aiming at the Honggang oilfield with higher salinity of 11948.4 mg/L, where petroleum carboxylate could reach ultralow oil-water interfacial tension with and without weak alkaline Na₂CO₃, the effects of mixed micelles formed from petroleum carboxylate and petroleum sulfonate on interfacial activites of the combined system were investigated. The research results showed the interfacial activities of petroleum carboxylate whose average carbon number is close to the carbon number of sodium oleate, were much better than that of sodium oleate, because petroleum carboxylate was composed of molecules with different length of carbon chain and having different hydrophilic lipophilic balance (HLB) values, respectively, and having different insertion depth of molecules at oil-water interface subsequently, the distance of polar groups became far and polar group’ s repulsion reduced. As a result, their mixed micelles arranged more closely, the interfacial adsorption increased and the interfacial activity was better than that of sodium oleate which composed of identical molecules. The combination effect of petroleum carboxylate and alkyl benzene solfonate in Honggang oilfield with the higher salinity was good also. the difference of HLB value between carboxylate and sulfonate made the mixed micelles from petroleum carboxylat/petroleum sulfonate combined system arrange more closely, so that the interfacial activity of the system was better, and the time to reach the initial ultralow interfacial tension (IFT) was shorter than that of a single surfactant.