Abstract: On the basis of the detailed investigation on a building site, the expansive and compressive mechanism of the third clay and fourth one in the site is discussed and appraised by the microcosmic analysis methods, such as X-diffraction, differential heat analysis, transmission electron microscope etc, and general indoor experimental methods of hydrological properties. The fracturing pattern of a building on expansive soil foundation and the geotechnical conditions of building crack are expounded, and their internal and external factors are analysed.