Abstract: The Hongsibu Basin is located at the junction of the forefront of the arc-shaped structural belt on the northeastern margin of the Qinghai–Tibet Plateau and the western part of the Ordos Plateau. The regional ecological environment is fragile, yet the lack of complete outcrop profiles due to extensive surface coverage has long hindered research into the evolution of the ecological environment. This study, based on the drilling sequence of Well ST1 in the Yaoshan Sag of the Hongsibu Basin, employs optically stimulated luminescence dating to establish a Late Pleistocene stratigraphic chronology. Additionally, it systematically analyzes the major and trace element compositions of 70 samples and reconstructs the climatic and environmental evolution processes since the Late Pleistocene based on geochemical indicators. The study suggests that the boundary age between the Malan loess and the Salawusu Formation is approximately 67.57 ± 7.88 ka, indicating a transition from an ancient lake to an aeolian environment, accompanied by a gradual deterioration of the regional ecological environment. The geochemical characteristics of elements also recorded the climatic and environmental changes during this period. Geochemical indicators such as the CIA value, Al₂O₃/Na₂O, K₂O/Na₂O ratios, and clay mineral content indicate a paleoclimatic evolution process of arid-warm-arid in the Hongsibu Basin since the Late Pleistocene. Rb/Zr and Sr/Ba values reflect a closed lake basin environment during the deposition of the Salawusu Formation in the Hongsibu Basin. The depth of the sedimentary water body increased from the first section of the Salawusu Formation, reaching its maximum depth in the second section, and gradually became shallower in the third section until the complete disappearance of the ancient lake during the deposition of the Malan loess. The research results provide a new basis for the sedimentary environment evolution of the Late Pleistocene in the northeastern margin of the Qinghai–Tibet Plateau from the perspective of elemental geochemistry.