The seismic subsidence is one of major disasters in loess areas in China. The deformation mechanics of loess structure under earthquake loading are analyzed. By using electronic-microscope scanning, microstructure and grain shape of Q₃ loess, which has seismic subsidence property, are studied. It indicate that the microstructure of loess has great influence not only to the seismic subsidence coefficient, but also to the critical dynamic stress. Through the quantitative test analysis of pore content, the influence of middle-large pores on loess seismic subsidence is discussed and some items of loess samples' microstructure are compared and analyzed.