Abstract: In order to study the variation of suction and saturation in natural slopes of unsaturated expansive soil and the rules of rainfall influencing suction and saturation, matrix suction and saturation in situ near a highway at Xiangfan in Hubei province were measured with thermal conductivity sensors and moisture probes for two months. Soil-water characteristic curve (SWCC) is obtained from the data measured in situ, from which we can acquire air entry value (AEV) and residual saturation (RS). Permeability coefficient is derived from SWCC. At the same time, reasons and rules of suction variation with depth are explained.