The lunar south pole is characterized by complex illumination conditions, extensive shadowed regions, and rugged topography, which make high-resolution terrain reconstruction from single-view imagery particularly challenging. To address this problem, this paper proposes an Illumination-Prior-Based DEM Reconstruction framework (IPDR) for single-view lunar terrain reconstruction under initial DEM constraints. The proposed method jointly exploits monocular image features, geometric prior information from the initial DEM, and illumination guidance derived from solar geometry. Experimental results on representative lunar south polar regions show that the proposed method achieves more stable and geometrically consistent reconstruction results than the compared methods. Large-area and local comparisons demonstrate improved terrain continuity and local morphological recovery, while ablation experiments verify the complementary roles of the initial DEM and the illumination prior. Additional experiments under different solar azimuth conditions and on ShadowCam imagery further show that the proposed framework maintains good robustness and adaptability. These results demonstrate the effectiveness of the proposed method for high-resolution lunar south polar terrain reconstruction from single-view imagery under initial DEM constraints.