To improve the durability and stability of subgrade on service, we aimed to research on a scientific method for filling subgrade with completely decomposed granite soil under the hot and humid condition in south. Wet heavy compaction test and California bearing ratio test were conducted on the completely decomposed granite. The results show that the moisture content of completely decomposed granite was more similar to natural moisture content than the optimum moisture content under the maximum bearing capacity. Swelling experiment was carried out to obtain the regularity of density decay by variation of initial moisture content. The modified consolidation test was applied to analyze the deformation properties on the maximum bearing capacity and the maximum dry density status respectively. The results show that compared to maximum dry density condition, the resistance of deformation and the stability on the maximum capacity status was enhanced. Moreover, the experimental subgrade filling with completely decomposed granite was constructed on the maximum bearing capacity status. The in-situ bearing plate test and compactness test were conducted, and the results show that the compactness could totally meet the demand of lower road bed which is the 94% compactness region. To meet the design modulus of the subgrade that the pavements demands, we proposed a method of stiffness compensation based on principle of equivalent deformation to ensure the stiffness and the durability of the completely decomposed granite subgrade effectively.