Abstract <p>Unsaturated soil mechanical and hydraulic properties are hard to characterize accurately, as they depend on pore structure. The air-entry value (AEV) is one of the key indicators reflecting the soil’s micro-pore structure, and it plays a crucial role in research related to unsaturated soil parameterization models and hydraulic measurement techniques. This paper first discusses the factors affecting the AEV and summarizes the methods for determining it. Then, it systematically reviews the soil-water characteristic curve, permeability coefficient prediction models (including those under soil deformation conditions), and unsaturated soil strength prediction models, all of which use the AEV as a key parameter, highlighting the significant role of AEV. Finally, the application of high AEV materials in the measurement techniques of unsaturated soil hydraulic properties is introduced. This paper provides a comprehensive summary of research on the AEV as a key parameter, which can facilitate the study of the relationship between the microstructure and macro hydraulic and mechanical properties of unsaturated soils through the use of AEV as a bridge.</p>