Surfactants are extensively used in chemical flooding due to their excellent ability to reduce oil-water interfacial tension，alter rock surface wettability，and promote oil-water emulsification. It was previously believed that the middle phase microemulsion could only occur at high surfactant concentrations. Therefore，the evaluation of surfactants for chemical flooding mostly depends on interfacial tension measurement. The role of microemulsion phase behavior technology in the screening and development of surfac‐tants for tertiary oil recovery is neglected. Thanks to the technological advancement in surfactant development in the past two de‐cades，ideal middle phase microemulsions have been achieved by crude oil systems with low acid values at low surfactant concentra‐tions. As a result，it is imperative to use microemulsion phase behavior technology to guide the development of new surfactants and the performance optimization of the composite system. This paper started with the concept and classification of microemulsions and systematically introduced four emulsification types of microemulsions，namely Winsor Ⅰ，Ⅱ，Ⅲ，and Ⅳ. The formation mecha‐nism of the four types of microemulsions was described from the aspects of oil-water interfacial tension，surfactant molecular struc‐ture，and micellar morphology. The mechanism involved instantaneous negative interfacial tension theory，duplex film theory，R ra‐tio theory，and geometric arrangement theory. The microemulsion phase behavior technology was introduced in detail in terms ofphase pattern identification，composition calculation of oil，water，and surfactant in the microemulsion phase，and laboratory op‐eration. In terms of processing results of phase behavior experiments，it was introduced in detail how to calculate the solubilization index of the oil and water phases in the microemulsion through their respective volume changes and how to calculate the interfacial tension between the microemulsion phase and oil and water through the solubilization index，so as to determine the optimal salinity of the composite system. This paper hopes to attract people’s attention to the microemulsion phase behavior technology and promote its application in the research and development of surfactants for high-efficiency tertiary oil recovery，the study of oil displacement mechanism，and the optimization of composite system formulation，thus improving the technical level of compound flooding in China.