Understanding how climate warming affects plant diversity in montane ecosystems is one of the central issues in studies of past global change. However, this issue remains inadequately understood, especially within biodiversity hotspots. Here, we present a multi-proxy analysis– including pollen, grain size, and loss-on-ignition–conducted on a 170 cm-deep soil section from the Hengduan Mountains of southwestern China to elucidate how vegetation and plant diversity have responded to climate change over the past 14.2 ka. Our results show that 1) Solar insolation and the Indian summer monsoon influenced climate variability in the region, triggering an alternation between mixed coniferous and broad-leaved forest (mainly Pinus, Abies, and evergreen sclerophyllous Quercus) and lakeside herbaceous vegetation. 2) Plant diversity exhibited a complex response to three natural warming events – the Bølling-Allerød warm period (14.2–12.8 cal. ka BP), the end of the Younger Dryas event (12.8–11.5 cal. ka BP), and the Holocene Climatic Optimum (6.7–5.3 cal. ka BP). Specifically, species richness increased, while the Simpson index, Shannon-Wiener index, and evenness decreased. 3) Climate warming influenced plant diversity by modifying vegetation structure and composition, leading to a more uneven distribution of species in the communities. These findings not only offer a scientific basis for biodiversity conservation and sustainable development in the montane regions of southwestern China under future warming scenarios, but also serve as a valuable reference for examining deep-time plant diversity responses to climate warming in similar montane ecosystems worldwide.