Abstract: The soil-water characteristic curve (SWCC) is the main constitutive relation to reveal the properties of unsaturated soils. However, few researches on soil-water characteristics of unsaturated soils in high-temperature environment, especially over 100˚C, have been reported. The SWCC of soft clay from Shanghai at high temperature is investigated using the vapor phase technique. Different temperatures (20˚C, 105˚C, 150˚C and 200˚C) are adopted to dry the soft clay for 4 h before testing the SWCC. In addition, the microstructures of soft clay before and after high temperature are characterized by scanning electron microscopy (SEM). The results show that the SWCC of soft clay decreases gradually as temperature increases to >100˚C. Similarly, the water-retaining capacity, suction force and intake value of soft clay decrease with the increase of temperature. These results are mainly attributed to the variation of the microstructure of soft clay according to the SEM results. The number of pores of soils decreases, while the pore size of a small number of pores increases at high temperature, which accounts for the variation mechanism of SWCC of soft clay at high temperature. In a word, the change of pore number and pore structure with the increase of temperature is the fundamental reason for the change of water characteristics of soft clay with the increase of temperature.