Granite residual soil is often used in the filling of highway roadbed because of its high strength; nevertheless in rainy season, the quality of filling would be affected markedly by rainfall due to granite residual soil's strong permeability and poor anti-scour ability. By adding red clay into granite residual soil, the influence of red clay content on permeability coefficient and unconfined compressive strength of granite residual soil was examined via permeability test and unconfined compressive strength test. On the basis of indoor test, comparative study was also carried out with specific filling project. Results revealed that with the increasing of red clay content, the permeability coefficient of granite residual soil declined while unconfined compressive strength augmented. The optimum content of red clay in granite residual soil is 40%. In actual project, two vehicles of granite residual soil mixed with one vehicle of red clay could achieve good filling effect. The research findings could be references for the filling and reinforcement of project construction in granite residual soil area.