Abstract: The "water vapor adsorption isotherm - TG, DSC" method is proposed to define and investigate the type of bound water of typical Anyang weak expansive soils. A comparative analysis of the distribution characteristics and amount of bound water is made between the original soil and ISS-soil. From the perspective of water molecular structure vibration and based on the theory of Fourier transform infrared spectroscopy, the rationality of the proposed method is also qualitatively validated. Finally, the effect of ISS on the bound water is interpreted in terms of the change of the clay mineral composition and surface physicochemical property. The test results indicate that the distribution state and property of the boundwaer will alter at specific relative humidity interval. According to the TG - DSC curve, the bound water can be divided into three types at different elevated temperature ranges, i.e., strongly bound water, transitional bound water and weakly bound water. After modification, the strongly bound water remains consistent while the transitional and weakly bound water significantly decreases. The ISS weakens the capacity and ability of hydration of outer surface and interlayer of expansive clay mineral and hence effectively inhibits the swelling potential induced by the soil hydration.