Quantifying the interactions between topography, climate and plant diversity within one of the world's biodiversity hotspots, the southeastern margin of the Tibetan Plateau, remains elusive due to few reliable quantitative paleoelevation reconstructions, precise geological age constraints and well-preserved plant fossils. The Lühe Basin, on the southeastern margin of Tibetan Plateau has yielded abundant plant fossils with a U-Pb age of 33–32 Ma, providing an opportunity to estimate the elevation of this region and plant diversity at that time. Fossil leaf physiognomy was used to reconstruct the paleoclimate and the paleoelevation of the basin was derived from moist enthalpy. The results show that the Lühe Basin, had attained it's present elevation (1.7 ± 0.9 km) by the early Oligocene and, compared to now, experienced a humid subtropical climate with a wetter dry season and lower precipitation seasonality in an overall wetter precipitation regime (1748.5 ± 606 mm). This was accompanied by a greater seasonal range in temperature, although the mean annual temperature (14.9 ± 2.3 °C) was similar to that of today (15.6 °C). Combined with previous studies, we conclude that the appearance of the modern flora across the southeastern margin of the Tibetan Plateau had started by the early Oligocene, corresponding with the establishment of modern topography at that time.