In order to study the influence of the component in Changqing crude oil on the interfacial tension of surfactant solution， the crude oil fractions were obtained by four-fraction separation method and NaOH/ethanol mixture extraction. The interfacial tension between crude oil fraction and betaine surfactant solution was measured. The results showed that single fraction，such as saturate，aromatic and asphaltene had low contribution to the formation of ultra-low interfacial tension. On the other hand，the interfacial tension between non hydrocarbon component and betaine surfactant could reach 4.82 × 10^-3 mN/m. The equilibrium interfacial tension between petroleum acid in non hydrocarbon component and surfactant was 0.347 mN/m，while the interfacial tension between petroleum alkali and surfactant decreased rapidly and then increased to about 0.1 mN/m. Single petroleum acid or petroleum alkali could not effectively decrease the interfacial tension to low value，furthermore they had the opposite interface characteristic. The interfacial tension between the compound system of petroleum acid and petroleum alkali and the surfactant was 4.70×10^-3 mN/m. The synergistic effect of petroleum acid and petroleum alkali caused the interfacial tension between crude oil and betaine surfactant to reach ultra-low value.