Granite residual soil, distributed widespreadly in China Southeast coastal region, is a commonly soil mass used for the urban construction projects. Using readily available the local soils as filling materials for high-fill embankments and slopes is an most costing economic strategy. For high-fill slopes, using geogrid reinforced soil technology can improve soil shear strength and enhance the slope stability. Firstly, direct shear friction tests were done to investigate the frictional characteristics of the reinforcement-soil interface. Furthermore, in order to study the impacts of dry densities and number of reinforcement layers on the shear strengths of reinforced granite residual soil, triaxial consolidation and drainage shear tests were used to investigate shear strength and deformation of both saturated granite residual soil and saturated reinforced granite residual soil separately. The testing results show as follows. The specimen normalized strength coefficients corresponded to different reinforcement-layer number are overall greater than unity, this shows that embedded reinforcing geogrid into the soil body can both enhance soil shear strength and decrease soil body deformation effectively. The normalized strength coefficient does not have a linear relation with reinforcing-layer number increasing, and effect of the second reinforcement layer tends to be weaken on soil shear strength increasing. The normalized strength coefficient increases as dry density deceasing and reduces as confining pressure increasing. The geogrid primarily enhances soil shear strength through increasing the apparent cohesion of reinforced soil obviously, the quasi-cohesion increment calculating formula is proven to be reasonable for the saturated reinforced granite soil through comparing theoretical calculating results against test results, and the second reinforcement layer only contributes slightly increasing to the quasi-cohesion increment.