The thick alluvial, lacustrine, and marine sediments deposited in the Bohai Sea coastal plain hold valuable information about Cenozoic climate change, global sea- level fluctuations, sediment provenance, and regional tectonics. However, reconstructing long- term paleoenvironmental changes in this area has been hindered by the scarcity of well- dated, deep sediment cores. In this study, we present geochemical and palaeobotanical records from two well- dated boreholes (G2: 1226 m and G3: 905 m) that extend back to the late Miocene in the Bohai Sea coastal plain. Our results indicate that the CIA and ratios of Al/Na, Al/K, and Al/Si are good indicators for the degree of chemical weathering. Intense chemical weathering is observed between 8. 5 Ma and 3. 6 Ma, coinciding with a relatively warm and humid climate as revealed by sporopollen records from the G2 borehole. Since 3. 6 Ma, a decrease in chemical weathering intensity and an increase in conifers in the palynological assemblages indicate a gradual cooling trend, consistent with ongoing high- latitude cooling and the strengthening of the East Asian winter monsoon during this interval. A sharp decline in chemical weathering intensity is observed since ~1. 5 Ma, deviating from the gradual trends observed in deep- sea δ18O records and loess CIA values. This discrepancy is attributed to the influx of dry and cold climate- derived sediments from the Loess Plateau into Bohai Bay, leading to reduced chemical weathering intensity. This interpretation is supported by our previous detrital zircon provenance analysis, which revealed a shift in sediment sources around 1. 5 Ma. We propose that tectonic movements, coupled with persistent dry and cold conditions, lowered the regional water level datum and intensified headward erosion in North China since ~1. 5 Ma. This provided the driving force for the Yellow River to incise through the Sanmen Gorge and enter the North China Plain, contributing eroded loess material to the Bohai Sea coastal plain.