Abstract: The accurate determination of water content for soils is particularly important for studying the soil-water characteristics of unsaturated soils with high suction range. At present, the oven temperature is set at 110℃ in the standards of soil testing method at home and abroad. Few literatures have studied the rationality of the temperature setting. Here, seven different types of clays, Na and Ca- Gaomiaozi bentonites, Shanghai soft clay, kaolin and Taizhou coastal soft clay, are analyzed by derivative thermogravimetry (DTG) and thermogravimetric analysis (T-G) with synchronous thermal analyzer. At the same time, their mineral compositions are tested by the X-ray diffraction (XRD). The test results show that the higher the mass percentage of montmorillonite minerals in soil samples, the higher the temperature of losing adsorbed water. When there is no montmorillonite mineral composition in soil samples, the temperature of losing adsorbed water is lower. It is considered that the oven temperature should be raised from 110 to 150 ℃ for measuring the water content of soil samples with high montmorillonite content.