Abstract: Granite residual soil is abundant in the humid tropics of southern China. It has extensively been used as fill materials in various applications, such as back-filling materials for pavements. Some subgrade diseases are related to the compaction degree and the moisture content of the compacted granite residual soil. On the basis of analyses of the characteristics of the granite residual soil in Rong County Guangxi, a series of direct shear tests are carried out on the compacted samples with three moisture contents (optimum moisture content of the standard compaction test, on the wet side of the optimum moisture content, and on the dry side). The microstructure and fabric characteristics of these compacted samples are analyzed by scanning electron microscope and mercury intrusion pore size distribution measurements. The experimental results show that there samples have the same stress-strain curve shapes with no pronounced peaks. The optimum sample has the highest shear strength, the three samples have nearly equal internal friction angle, the optimum sample has the highest cohesion, and the cohesion of the wet sample and the dry sample decreases by 66.4% and 43.1%, respectively. The wet sample has aggregate structure and open structure and exhibits a doublet pore-size distribution with the highest cumulative pore volumes, while the optimum sample has a matrix structure with parallel alignment of mineral grains. The dry sample has a piece of frame structure with the smallest cumulative pore volumes. The microscopic features of these three compacted samples are explained by the difference of the shear strength accurately.