In order to clarify the law of petroleum acid on the interfacial tension and emulsifying performance of binary oil displacement system（SP），two kinds of petroleum acids and oil with acid removed were separated from crude oil in compound flooding test area. The carbon number distribution and structure of two petroleum acids were tested，and the influence on dynamic interfacial tension and emulsion stability of SP were studied. The results showed that the relative molecular mass 1# and 2# petroleum acid was 242 and 312，while the carbon number distribution was distributed between 10—15 and 10—25，respectively. The 2# petroleum acid with mass concentration of 700 mg/L could reduce the equilibrium interfacial tension（IFT_eq）between oil and water to 0.021 mN/m，while 1# petroleum acid with the same concentration could only reduce IFT_eq to 0.095 mN/m. The mixed acid had a positive synergistic effect on reducing interfacial tension. The combination of two petroleum acids could achieve ultra-low oil-water interfacial tension in SP. The stability of acid-containing emulsion system was related to the dynamic interfacial tension. The final stability of the oil-water emulsion system containing 2# petroleum acid was higher due to the low IFT_eq. Through the reverse analysis from the performance to raw materials of SP，it was pointed out that the sulfonate should be prepared by adding petroleum acid to raw oil，which increased the emulsification performance of SP and obtained a better effect of reducing interfacial tension.