Abstract: In order to better understand the effects of climatic aridity on terrestrial vegetation types and lake aquatic plant communities within the Northwest China since the Eocene, we systematically studied the organic geochemical characteristics of the drill core samples from the Yiliping area of the Qaidam Basin, including total organic carbon content, Rock-Eval pyrolysis derived indices and saturated hydrocarbon chromatography parameters. The hydrogen index (HI), and a series of n-alkane and isoprenoid alkane proxies (ACL_15-35, ACL_27-33, P_aq, C₁₇/C₂₃, C₃₃/C₂₇, R₂₂, Pr/Ph) were used to deduce the origin and evolution of the organic matter types within the lake, the change of the terrestrial vegetation types, as well as their relationship with the climatic changes. The results show that the lake environment and ecology was influenced by the Eocene-Miocene global climate change. During the depositional period from the upper member of the Xiaganchaigou Formation (E₃²) to the Shangganchaigou Formation (N₁) (about 38~22 Ma), the aquatic plant change from algae-dominance to submerged plants-dominance, while the terrestrial vegetation types remain unchanged. During the depositional period of the Xiayoushashan Formation(N₂¹) (about 22~15.3 Ma ), the climatic aridity and the shrinking of the lake have led to significant increase in salinity, and a strong contribution of terrestrial and emergent plants input, and grass replaced woody plants as the dominant plant community on the perimeter of the lake.