<p>Over the past millennia, the Eurasian Steppe has undergone extensive population migrations and profound social transformations. Within this context, the subsistence strategies of ancient Eurasian Steppe societies, particularly the evolution and driving mechanisms of their dietary structures, have attracted significant scientific interest. As a key region in the central-eastern Eurasian Steppe, Xinjiang serves as an important window for investigating the aforementioned questions. In this study, we present stable carbon and nitrogen isotope analysis of 75 human and 5 animal bone samples from the Baiyanghe cemetery in the northern Tianshan Mountains, Xinjiang. Combined with radiocarbon dating, these data elucidate the dietary patterns of local populations from the Bronze Age to the Jin-Tang periods. We also integrate published stable isotope data from nearly 1,500 human and animal bones across the central-eastern Eurasian Steppe to systematically reconstruct regional dietary evolution since the Bronze Age and investigate their driving factors. The results indicate that the Baiyanghe population during the Late Bronze Age primarily consumed C₃-based foods, including meat and dairy products from domesticated herbivores, as well as possible C₃ crops such as wheat and barley; From the Early Iron Age to the Han Dynasty, local diets were characterized by a significant increase in the consumption of C₄ crops (likely dominated by broomcorn millet) and animal proteins from both pastoral and aquatic resources. This pattern then reversed during the Jin-Tang periods, with diets shifting back to a predominance of C₃ resources. Cross-regional comparative analysis further reveals a synchronous dietary shift across multiple areas of the central-eastern Eurasian Steppe: a coordinated increase in C₄ crops and pastoral product consumption from the Early Iron Age to the Han period, followed by a marked decline in C₄ crops and a return to C₃ dominance from the Han to the Jin-Tang period. Integrating paleoenvironmental and archaeological evidence, we propose that the large-scale, synchronous dietary shifts across the central-eastern Eurasian Steppe from the Bronze Age to the Tang Dynasty arose from combined climatic, social, and technological factors. Specifically, a warm, humid climate and intensified interregional exchange among pastoralist groups primarily drove the initial transition toward C₄/herbivore-based diets. This pattern then gave way to a return to C₃ dominance, co-driven by a cooler, drier climate and advances in grain processing technology. This study advances our understanding of dietary dynamics and human-environment interactions in the Eurasian Steppe, providing a crucial empirical reference for future research.</p>