Abstract: The soil-water characteristic curve is very important for studying the physical and mechanical characteristics of unsaturated soils. Strength, volumetric strain and permeability coefficient, and even moisture-distribution above ground water surface can be determined by the soil-water characteristic curve. A mathematical model of soil-water characteristic curve is established, based on soil, structure and the shape of soil-water characteristic curve. Mathematical models of soil-water characteristic curve differ due to varying physical and mechanical properties of unsaturated soils. It is, thus, necessary to establish universal expression of soil-water characteristic curve. In this paper, mathematical models of soil-water characteristic curve are divided into four types based on the patterns of mathematical models. An universal mathematical model with unified functional pattern is derived. The soil-water characteristic curve of loessal soils in north Shaannxi loess plateau, is used to verify the universal mathematical model.