The Nihewan Basin in North China is one of the most important early archaeological sequences for our understanding of the ecological ranges exploited by hominins and how their adaptive behaviors were shaped by local, regional, and global environmental fluctuations. This study contributes to the topic by conducting stable isotope (δ¹³C, δ¹⁸O) analyses of mammalian tooth enamel from several Early Pleistocene assemblages in the Nihewan Basin dated to ca. 1.2–1.1 Ma, and exploring the links between hominin behavioral adaptations and ecological changes during the Mid-Pleistocene Climate Transition in Northeast Asia mainland. Our isotopic analysis of bulk samples (n = 215) indicates that the Nihewan Basin regional environment underwent significant changes, from landscapes dominated by wet and closed pure C₃ vegetation no later than 1.2 Ma, to those dominated by dry and open C₃/C₄ mixed vegetation during 1.2–1.1 Ma. Additionally, the isotopic analysis of sequential samples (n = 16) suggests seasonal variations. These isotope results interpreted in the context of hominin behavioral traits indicate that hominins exhibited cognitive and technical flexibility in the face of complex and unstable regional ecological settings, suggesting that they adjusted to the environmental fluctuations characteristic of high-latitude arid/semi-arid regions such as the Nihewan Basin over or prior to the Mid-Pleistocene Climate Transition.