Global cooling, the uplift of the Tibetan Plateau, and local topography have long been considered the crucial factors influencing paleoclimate change in the northeastern Tibetan Plateau since the Cenozoic era. However, the leading role played by each of these factors remains unknown. In order to gain a better understanding of paleoclimate change and its controlling factors, we conducted a study on the micro- morphology and relative content of clay mineralogy in the sedimentary sequence of the Xunhua basin in the northeastern Tibetan Plateau, spanning from the Middle Miocene to the Early Pliocene. Our results show that the clay minerals deposited in the Xigou section of the Xunhua basin are comprised of illite, smectite, kaolinite, and chlorite. Illite is the dominant clay mineral, followed by smectite, while the kaolinite and chlorite contents are relatively low. Combined with the content variation of herb- bushwood, coniferous forest, and broadleaved forest in the palynological assemblage, as well as the clay minerals, organic carbon isotope, sedimentary rates of sedimentary sequence between Middle Miocene and Early Pliocene in the Xigou section from the Xunhua basin, and δ18O isotope record from the global deep- sea, the paleoclimate evolution in the Xunhua basin can be divided into three stages: Ⅰ- relatively cold and dry period (14. 6~12. 7 Ma); Ⅱ- relatively warm and humid period (12. 7~8. 0 Ma); Ⅲ- cold and dry period (8. 0~5. 0 Ma). Based on regional geological evidences, the climate cooling and drying event that occurred between 14. 6 Ma and 12. 7 Ma aligns with a global cooling period triggered by the expansion of the Northern Hemisphere Ice Sheet. Our inference of sustained elevated humidity observed in the Xunhua basin at 12. 7~8. 0 Ma is considered to reflect changes in regional rainfall patterns related to orogenic uplift. The Jishi Mountain range reached a critical threshold elevation at around 12. 7 Ma, becoming an orographic barrier to Westerlies- transported moisture and thus enhancing intensified precipitation within the Xunhua basin. However, by around 8. 0 Ma, further uplift of the Tibetan Plateau caused this orogenic barrier to obstruct the East Asian monsoon and westerlies, resulting in the aridification of the Xunhua basin.