Abstract: Loess is a typical structural soil, and its seismic subsidence is mainly due to the internal microstructure and external load. Seismic subsidence of loess is a congregated macroscopical phenomenon of microstructure with the external load in different time and space. The distribution of loess particles and pores is obtained through electron microscope scanning experiments. The structural damage model is established by means of the statistical microscopic damage mechanics and structural mechanics. It explains the inherent damage mechanism of seismic subsidence, and the complete constitutive relation is obtained. The results indicate that the seismic subsidence of loess results from the damage of particular trellis pores and particles falling into trellis pores. It appears that there's a great relation between the seismic subsidence of loess and the dynamic stress and the distribution of particles and pores.