Subgrade is the main body of road. The subgrade bears its own weight and the static load of the pavement, and the kinetic load caused by people, livestock, and vehicle walking on the pavement. This paper reviews the stability of the rural road subgrade and its influencing factors in terms of soil, engineering geological conditions and hydrogeological conditions. The stability of the subgrade filling soil depends largely on the soil cohesion and the internal friction angle, while the bearing capacity mainly rests upon the water content and compactedness. There are so many geological factors influencing the subgrade stability, such as rock types, weathering degree, fractured degree, the rock trend, tendency, dip angle, stratification, and thickness, with or without dissection or water to soften the interlayer, with or without faults or other undesirable phenomena along the road. Not only does the water permeability damage the soil, but also it can reduce the soil shear strength. These conclusions have great theoretical value for the rural road subgrade design in the hilly-mountainous area.