During rainy seasons, flood damage occurs easily to the riverside highway subgrade in mountainous areas, threatening the driving safety. Therefore, to master the environmental characteristics and the disaster-causing patterns is of great significance for the radical treatment of flood damage. On the basis of theoretical analysis and field survey, this work, taking Qinling-Bashan (hereinafter referred to as Qin-Ba) Mountainous Area in Southern Shaanxi Province for example, mainly deals with the types and the causes of the flood damage of riverside highway subgrade and hence the corresponding prevention measures against it, which may provide a basis for the design of riverside highway and also the prevention of flood damage in rainy season.