At present, the LOD modelling of vector and terrain data is individual, resulting in geometric and topological inconsistencies in simplification processes. This study proposes the "Bending Adjustment Index (BAI)" to improve the 3D_DP algorithm, called the improved 3D_DP algorithm, and putsforward a new unified LOD model for river network and DEM in three-dimensional space. The DEM data is extracted into a 3D discrete point dataset, the river line vector data is also converted into a discrete point dataset, assigned with elevation attributes, and merges with the DEM’s 3D discrete points. The merged point dataset issequenced based on the importance of each point, which is computed by the improved 3D_DP algorithm. The order of deleting points is determined by the sequence and the corresponding model spatial errors arecalculated with the deletion ofpoints. Then, the DEM’s 3D discrete points are constructed a Delaunay network with the river line as a mandatory constraint condition. The required triangulation is called in real time with the change of sight distance depending on the simple correspondence between screen projection error and model space error, and the unified LOD model for river line vector and DEM is established. The results show that the river's overall shape and the t errain's main features can be reserved under the same simplified factor based on the BAI improved 3D_DP algorithm. The unified LOD model for the river network and DEM is feasible under the importance sequence of merged point dataset by the improved 3D_DP algorithm. Under the proper operation of data blocking, the rendering frame rate can meet practical application requirements.