Detailed analysis of whole-rock geochemistry, clay minerals, sedimentary color, and pollen in the Dahonggou section, northeast of the Qaidam Basin, are investigated, and the results suggest an intense weathering in the source area during the middle Eocene (∼48.5–40.5 Ma), indicating a warm and humid condition. The distinct decrease of chemical weathering degree in source regions began at ∼40.5 Ma, which is in agreement with the distinct decrease in redness of sedimentary sequences and the disappearance of thermophilic elements in pollen records. This 40.5 Ma cooling event extent demonstrated evidence for an intensification of central Asian aridification, which could be attributed to attainment of high elevations in southern-central Tibet and retreat of the Paratethys from central Asia in the late Eocene, reducing moisture transport to the Qaidam Basin.