Abstract: In arid and semi-arid regions, such as in northwestern China, unsaturated soils at high suction or over a wide suction range are widespread in various geotechnical designs. Studies of the soil water retention behavior are fundamental to a comprehensive understanding of the water flow, deformation processes, and shears strength for unsaturated soils. To investigate the soil water retention behavior of a silt over a high suction range, the suction is imposed on silt specimens using the vapor equilibrium technique, and the volume of irregular soil sample is measured using the Archimedean method. The soil water characteristic curves (SWCCs) with different initial void ratios at high suctions were simulated by the modified Van Genuchten’s model. The results show that (1) SWCCs in terms of water content versus suction relation with different initial void ratios are independent of the initial void ratio at high suctions. That is, soil water retention at high suctions is mainly related to adsorbed water content. (2) It was found that the fitting results of the existing equation based on Van Genuchten’s model can not accurately simulate the soil water retention behavior over a high suction range. A modified soil water characteristic equation is proposed by introducing a correcting function into the equation. (3) A modified relative permeability coefficient equation is proposed and it can be applied to predict the relative permeability coefficient with different initial void ratios. The predicted results are in good agreement with the measured values.