The United Nations Convention to Combat Desertification identifies Land Degradation Neutrality as a key 2030 Sustainable Development Goal, yet long-term, high-resolution desertification records are scarce in arid northern China. The Ulan Buh Desert (UBD), at the core of China's wind-sand belt, is a major source of aeolian sediments entering the Yellow River, affecting ecological security and socioeconomic development across the basin and influencing windbreak-sand fixation systems in the Beijing-Tianjin region. Here, we reconstruct 140 years of desertification along the northern and southern margins of UBD using annually resolved shrub tree-ring records, overcoming the temporal limits of remote sensing and historical archives, and examine the underlying driving mechanisms. Desertification declined along the northern margin, while the southern margin remained relatively stable. Variability was marked by high-intensity phases in the mid-twentieth century and low-intensity conditions since the late 1980 s, with periods of divergence between margins. Both regions exhibited significant 2-10year cyclical variability and pronounced legacy effects. Mechanistically, desertification along the northern margin was primarily associated with climatic factors, particularly drought severity and precipitation variability, whereas the southern margin was more strongly associated with human activity, with climate as a secondary factor. Teleconnected climate modes modulated desertification dynamics in a phase- and frequency-dependent manner. This study fills a critical gap in long-term, high-resolution desertification reconstructions in UBD and provides a robust foundation for regional mitigation and ecological restoration strategies.