To study the weakly expansive clay obtained from a slope along Wuhan—Shiyan expressway in Hubei Province, soil-water property tests and some unsaturated triaxial tests with suction control were conducted, and the soil-water retention curve (SWRC) and unsaturated shear strength of this soil were obtained. Results show that the air-entry suction and the residual degree of saturation of the tested soil are 106kPa and 8%, respectively. The boundary effect zone and the transition zone can be identified on the desorption curve, but the residual zone is not so obvious. The unsaturated shear strength increases as suction increases within the range of controlled suction in the test, and friction angle, φb, in the triaxial shear test is 17.6°. Based on the results, constitutive models for predicting the unsaturated shear strength using the SWRC were evaluated, and comparisons between prediction and measurement were made. It is concluded that for engineering purpose, the constitutive model should be carefully selected based on soil properties when predicting the unsaturated shear strength using the SWRC.