The main engineering diseases and types of damage to road subgrade in China and abroad are briefly introduced, and the most commonly used detection methods are summarized. The principles, ranges of application, and advantages and disadvantages of these detection methods are comparatively analyzed. In combination with subgrade treatment of the Jiangsu coastal highway, applications of the core drilling detection method, which may cause damage, and the geological radar detection method, which does not cause damage, are analyzed in detail, and the effects of the two methods for subgrade treatments are also evaluated. The results show that the geological radar method can be used in grouting tests in the vertical direction, and the pile boundary can be clearly distinguished. They also show that the core drilling method can reflect indices of the plastic-liquid limit, density, dry density, and water content of road subgrade quantitatively and intuitively, and has obvious advantages in local surveying.