Abstract: The surface water film of fine particles in cohesive soil is an internal factor that affects the physical and mechanical properties of the soil. In the classical soil mechanics, the water film on the surface of soil particles is generally divided into two layers: strong bound water and weak bound water, i. e., the so-called electric double layer model. The existence of weak bound water is the reason for the plasticity of soil, thus, the plastic limit is the boundary moisture content of strong bound water and weak bound water. The model perfectly explains the consistency change and related physical and mechanical behavior of cohesive soil. However, through the isothermal adsorption test, it is found that there is an adsorbed water film on the surface of the soil particles, which has an important impact on the physical and mechanical properties of the unsaturated soil in the high suction section. Therefore, this paper divides the water film on the surface of soil into five types: monolayer adsorbed water, multi-layer adsorbed water, strong bound water, weak bound water and free water. The L₂ loess specimen was taken from the topmost layer of Q₂ in Zhengning of Gansu Province and are used to conduct isothermal adsorption test, liquid limit test and plastic limit test, respectively, to achieve the internal minerals that make up the loess and quantitative characterization of the types of water film on the particle surface.The soil-water characteristic curves (SWCC) of the loess specimen are tested. Combined with the above test results, the relationship between these boundary moisture contents and the matrix suction is defined on the SWCC. When the water vapor pressure is very low, the suction on the surface of the soil particles comes from the electrostatic attraction between the dipole molecules of water and the ions on the surface of the particles, forming a single layer of the adsorbed water, and the thickness of the water film is one water molecule diameter. In the place beyond the water molecular diameter from the particle surface, the suction comes from the van der Waals force, and the dipoles of water are oriented toward each other, forming multi-layer adsorbed water. When the moisture around the soil particles increases, the unbalanced molecular attraction on the surface of the particles can attract more polarized water molecules, the bound water is formed around the adsorbed water, which is divided into strong bound water and weak bound water. The water outside the adsorbed water and the bound water film is free water.