Abstract: The specific surface area (SSA) is an important physical and chemical indicator of clayey soils, and choosing the appropriate test method is the premise to obtain the correct parameters. The SSAs of 5 different clayey soils are determined by the following three methods: methylene blue (MB) method, ethylene glycol monoethyl ether (EGME) adsorption method, and glycerin adsorption method, and are estimated by the empirical formula, air-dried moisture content method. The experimental results show that, for the non-swelling clay, the SSA calculated by the above 4 methods are approximately the same, while for the soil samples containing montmorillonite, the SSAs measured by the EGME method are smaller than those by other methods. After montmorillonite soil samples are saturated with different adsorption solutions, the soil samples are analyzed through X-ray diffraction experiments. It is found that the spacing of air-dried montmorillonite crystal layer d₀₀₁=15.6 Å, while that saturated by EGME, glycerol and MB is changed to 15.9 Å, 17.5 Å, 19.0 Å, respectively. The montmorillonite crystal layer saturated in glycerol or MB is obviously expanded, which is facilitative for adsorbed molecules to enter the surface of the crystal layer, and the molecules also cover the layer better. The statistics of the existing research results indicate that the SSA reflects the composite effects of composition of various hydrophilic clay minerals and the relative proportions. Therefore, the SSA is correlated with the liquid limit, plasticity index and activity and other physical indicators, and it is available to fit them in the form of a power function. If the soils contain swelling clay minerals, poor correlation between the SSA and the clay fraction will be found because the effects of type of clay minerals cannot be reflected by the clay fraction.