Abstract: Due to the difference of moisture content in the fine clay in aggregate gravel soil,the variation of microstructure in fine clay directly affects the permeability of aggregate gravel. Considering the influence of pore characteristics (scale,arrangement and pattern) in different moisture contents,the experiment was conducted to study the microstructure of the fine clay in aggregate gravel soil at a slope near Shuizuwan Tunnel by scanning electron microscope (SEM) and PCAS quantitative testing technology. The result show that:with the addition of moisture content,the porosity firstly increased and then decreased. When the moisture content increased to 12. 5%,the moisture content reached to the maximum. Simultaneously,with the addition of moisture content,the gas porosity gradually decreased and the small pores increased obviously,but the medium pore and large pore decreased. The pore orientation angle under different moisture contents were concentrated in 10°-30°,50°-70°,100°-120°and 140°-160°,and the less moisture content, the less pore orderliness. Entropy probability and form factor increased with the increase of the moisture content,then peaked with the moisture content from 7. 5% to 10%,then reduced with the increase of the moisture content. By analyzing the effect of the variation of the fine clay gravel soil microstructure parameters under different moisture contents on macroscopic flow characteristics,it is useful for understanding the formation of slope seepage field and the disasters of decreased strength index of landslide.