As a new type of surfactant for oil displacement, internal olefin sulfonates have attracted much attention in oil field in recent years. As tertiary oil recovery technology is applied to high?temperature and high?salt reservoirs, the problem of temperature resistance and salt resistance of surfactant emerges. The rotating drop method was adopted to investigate the effects of ionic strength of Na? and Ca2? on the oil?water interfacial tension reduction performance of two internal olefin sulfonates (A?18, I?20) with different degrees of branching. The experimental results show that the internal olefin sulfonates surfactants generally have good salt resistance. Hydrophobic alkyl branching can increase the interfacial activity of surfactants. The active fractions in crude oil can be mixed and adsorbed with surfactant molecules at the interface, which is an important factor governing the interfacial performance. The increase of ionic strength can weaken the electrostatic repulsion among surfactant molecules and increase the adsorption capacity of surfactants at the interface. Moreover, divalent calcium ions exhibit stronger molecular aggregation ability than monovalent sodium ions, further reducing the oil?water interfacial tension at high mass fraction.