Flooding containing alkali was commonly used in EOR. The interface-active components interacting with alkali in crude oil were introduced. The main active components were organic acids, non-acids and heavier substances which played a supporting role. Some crude oil also contained special trace components including siloxanes、amino acids and anilines which affected its alkaline emulsifying properties. The mechanisms of alkaline flooding were reviewed, such as reducing the interfacial tension, reversing wettability, emulsifying, increasing the viscosity of water phase, changing the rheology of crude oil, solubilizing the rigid film and thermodynamic analysis, as well as the influencing factors of alkali, salt, etc.