The Cenozoic sedimentary sequence of the Tianshui Basin preserves a key record of paleoenvironmental evolution on the northeastern Tibetan Plateau (NETP). This study investigates the Miocene Xiashan section to reconstruct paleosol development and environmental changes during 17-6.2 Ma, using macromorphology and micromorphology, together with sediment chromaticity and calcium carbonate content. Paleosols are dominated by massive, channel, and planar microstructures, with common clay coatings, FeMn oxides, and pedogenic carbonates. These features record pedogenic processes including decalcification, calcification, gleying, and rubification, reflecting alternating wet and dry conditions and fluctuating redox environments. At least seven climatic fluctuation stages were identified during 17-6.2 Ma. Humid intervals at 13.3-10.9 Ma and 8.3-7.2 Ma correspond to intensified pedogenesis and a stronger East Asian Summer Monsoon (EASM). Relatively arid conditions during 17-16 Ma, 9.8-8.3 Ma, and 7.2-6.2 Ma are marked by weaker soil formation and enhanced carbonate accumulation. Overall, the regional climate shifted from warm and dry to cold and dry conditions, with several short-term humid fluctuations superimposed on the long-term trend. Regional comparison shows that this evolutionary pattern is broadly consistent with other records from the NETP. After similar to 7.2 Ma, aridification intensified, likely driven by global cooling, Tibetan Plateau uplift, and Asian monsoon evolution. These results demonstrate that soil micromorphology is a reliable proxy for paleoenvironmental reconstruction and provide new constraints on Miocene climate change and EASM variability on the NETP.