In view of the problems in the traditional regular grid digital elevation model terrain feature line extraction method, such as the threshold is difficult to be adjusted quantitatively, the connection mode cannot be adjusted adaptively, and the type of terrain feature line is incomplete, an automatic extraction method of TIN-DDM (triangulated irregular network digital depth model) terrain feature line based on rolling ball transformation model was proposed. Based on the judgment criterion of a quantitative identification for terrain feature points associated with the critical rolling sphere radius, the concept of terrain shape boundary points was introduced, and the automatic extraction model of terrain feature lines based on subdivision unit was established according to the modeling idea of reverse engineering. Combined with the multi-scale expression characteristics of the terrain type judgment criteria and the terrain feature optimization model considering water depth values, an automatic extraction method of terrain feature lines that can be expressed at multiple scales and complete types was proposed. The experimental results show that compared with the classical surface water simulation method, this method can automatically extract complete, continuous, subdivided and multi-scale TIN-DDM terrain feature lines, and the generated terrain feature lines have higher terrain reconstruction accuracy.