Abstract: The variables of the soil-water characteristic curve(air-entry value, residual suction and slope at the inflection point) are very critical parameters in the strength theory, percolation theory and volume change behavior of unsaturated soil. It is often obtained by conventional graphical method, which is subjective and inaccurate. This study presents the methods for determining the variables of unimodal and bimodal SWCC based on VG model using the measured SWCC data of the four layers of undisturbed loess in Luochuan standard section. SWCC data were measured by filter paper method and best fitted using Van Genuchten's equation. Consequently the best fitting parameters were obtained, and the proposed methods were validated using the measured unimodal and bimodal SWCC data. The water evaporation test in the natural state was carried out. Then the evaporation residual saturation S_rzf was determined according to the relationship between the mass moisture content and the evaporation time. According to the relative errors between S_r1, S_r2 and S_rzf(S_r1 and S_r2 were acquired from the two methods for determining the residual state, respectively), the comparison was made between the two methods for determining the residual state. The results suggest that the proposed methods can produce reasonable variables and determine the residual state effectively related to the unimodal and bimodal SWCC.