As the world’s largest evergreen broadleaved forest (EBLF), East Asian EBLF houses high levels of biodiversity and endemism and is essential to regional carbon storage and cycling. However, how East Asian EBLF was shaped over time remains an open question. Here, we investigate the historical assembly of this biome by sampling 21 angiosperm clades (together encompassing 2028 species) that include evergreen and deciduous species. We show that the transition from deciduous to evergreen lineages, and in situ diversification and immigration of evergreen lineages in subtropical East Asia all experienced a dynamic process. Transition and immigration reached their first peaks and in situ diversification sharply increased at the Oligocene–Miocene boundary (OMB), and all of them reached their highest peaks in the late Miocene. Our results suggest that modern East Asian EBLF did not appear until the OMB and markedly deteriorated since the Pliocene, mainly driven by the evolution of Asian monsoon climate. This study suggests that East Asian EBLF has functioned as both museums and cradles for regional biodiversity, and serves as a transfer station for biotic exchanges between temperate and tropical regions, highlighting its great conservation value.