To expound the mechanisms responsible for the effect of electrolyte on the interfacial tensions (IFTs) of mixed solutions containing zwitterionic surfactant alkyl sulfobetaine (ASB) and anionic surfactant petroleum sulfonate (PS), the dynamic IFTs of ASB, PS and the mixed solutions with different ratios against hydrocarbons and crude oil have been investigated by the spinning drop method. The influences of NaCl on the dynamic IFTs have been detected. The experimental results show that the IFTs of PS solutions are greatly affected by electrolyte through the compression of the electric double layer. A compact film will be formed by PS molecules and ultralow IFT appears at optimized NaCl concentration. On the other hand, the IFTs of ASB with electric neutrality almost keep constant with an increase of NaCl. Synergism for reducing IFT may appear in ASB and PS mixed solutions due to the electrostatic attraction between them, which can be controlled by adjusting the size of PS through the variation of NaCl concentration. Ultralow IFTs will be produced during a wide concentration range at high NaCl concentration for mixed solutions. The studies in this paper are of great significance for the application of mixed surfactant solutions in enhancing oil recovery.