The formation and evolution of sandy lands are crucial for understanding the aridification processes of the Asian continent. However, limited research on the developmental processes and underlying forcings of the Songnen Sandy Land (SNSL) in Northeast China impedes a more profound understanding of the progressive eastward aridification processes in Asia interior. To address this gap, the grain-size characteristics and endmember (EM) modeling of the Harbin loess section are investigated to reconstruct the evolution of the SNSL since the late Middle Pleistocene and the forcing mechanisms behind it. The results reveal that three grain-size EMs can be identified in the Harbin loess: EM1 (12.7 mu m) represents atmospheric background dust, EM2 (45.6 mu m) indicates near-source stable suspended deposition, and EM3 (86.4 mu m) reflects short-distance transport driven by the East Asian Winter Monsoon (EAWM). The evolution of the SNSL is grouped into five stages: (1) initial dust emission stage (195-176 ka), during which the Songnen palolake began to shrink; (2) initial formation and stable development stage (176-142 ka), when the paleolake terminated; (3) rapid expansion stage (142-131 ka), where the SNSL, reached its maximum; (4) shrinkage stage (131-101 ka), in which the SNSL eventually reached its minimum; (5) slow expansion stage (101-32 ka), characterized by the reactivation of the sandy-land development, with persistent dust emission accompanied by deposition of the thickest loess layer. The evolution of the SNSL is driven by both high- and low-latitude processes, with global cooling and high-latitude ice sheet expansion playing key roles in promoting arid conditions in Northeast China. This study enhances our understanding of the relationship between the surface processes of the sandy-land landscape formation and climatic fluctuations in the Songnen Plain, while also illustrating a magnificent picture at the easternmost frontier for the eastward expansion of the Asian inland ariditification.