Abstract: The change of loessial soil microstructure before and after damages is analyzed by observing different cross-sections of specimen with a scanning electron microscope and a polarizing microscope. The results show that at pre-loading, the loessial soil has granulometric bracket contact microstructure with quartz, feldspar, illite, kaolinite and other minerals distributed randomly and astatically. The uniaxial compression process of the loessial soil can be divided into three stages. The first stage is a compacted phase, when microcracks close up and macropores become microcracks. The second stage is an initial damage phase which begins prior to peak stress. The soil microstructure turns to a lightly cemented mosaic microporous structure with slippages produced between particles. The original and newborn small cracks get wider and longer. The third stage is a rapid damage development phase which occurs after peak stress. The pores and cracks are interconnected, and the minerals have a pronounced directional characteristic. On this basis, constitutive damage equations are established by applying statistical damage theory using the characteristic of loessial soil micro-unit strength following the Weibull stochastic distribution, and considering the moisture content and damage threshold of loessial soil. Verification is carried out by taking the results of loessial soil uniaxial compression tests. The comparison of calculated results with test results shows that this model can properly describe the complete stress-strain test curves for loessial soil deformation and failure.