The microcosmic structure of the undisturbed soil sample done by lyophilization is tested. The distribution of pore size of Guangxi expansive soil are studied on the basis of fractal geometry by the result of mercury intrusion porosimetry; and the distribution of pore size is multifractal. The physical meaning of the inflexion of the curve is discussed; the result is confirmed by the photos of scanning electron microscope. The pore of expansive soil is sorted into the pore of soil group

the pore of particle in the soil group and the pore of montmorillonite crystal. The information of microcosmic pore cannot be gained by mercury intrusion porosimetry; the whole distribution of the pore size of expansive soil should be much accounted of.