In desert ecosystems, nebkhas play a pivotal role in mitigating desertification and sustaining ecological stability within oasis environments. However, an increasing number of these nebkhas are undergoing degradation, underscoring the need for a deeper understanding of their morphological evolution during this process. This study examines degrading Nitraria tangutorum nebkhas in two representative ecotones: the Minqin Oasis fringe and the desert-oasis boundary near Yanchi, Ningxia. A total of 233 declining nebkhas were identified and analyzed through field investigations, high-resolution digital orthophoto maps (DOMs), and UAV-derived digital elevation models (DEMs). Three degradation types were classified: collapse (C-d), layer-stripping (L-d), and wind-channel (W-d), with C-d and L-d accounting for 46.98 % and 48.70 % of the total number, respectively. The C-d nebkhas exhibit lateral collapses initiated primarily on the northern flanks, forming irregular, Yardang-like morphologies. The L-d nebkhas represent a transition from aeolian deposition to erosional dominance, characterized by synchronous reductions in length, width, and height. In contrast, W-d nebkhas develop as airflow laden with saltating sand incises erosion troughs that bisect the dune body. These results indicate that the degradation of Nitraria tangutorum nebkhas typically precedes or co-evolves with enhanced aeolian erosion, and provide new insights into the geomorphological dynamics and structural evolution of nebkhas on semi-fixed sandy and fixed sand land.