The discrepancies between model simulations and proxy reconstructions of global mean annual temperatures since the mid-Holocene warrant further investigation. Here, we present a high-resolution pollen record from Lake Ximenlongtan (XMLT) in the Hengduan Mountains, southwestern China, precisely dated to similar to 9.4-0 ka. The results show that tropical monsoon montane rainforest, characterized by the dominance of tropical and subtropical evergreen broadleaved trees, flourished around the lake during 9.4-5.5 ka. A notable vegetation shift occurred during 5.5-5.0 ka, when tropical taxa declined and subtropical taxa increased. This transition was followed by a reduction in both tropical and subtropical evergreen taxa until similar to 4.0 ka, indicating distinct cooling during 5.5-4.0 ka. Deciduous broadleaved trees and grasses expanded during this interval, indicating a pronounced drying trend. A spatial synthesis of published data from across the Hengduan Mountains revealed a significant ecological shift during 5.5-4.0 ka, characterized by the progressive loss of warmth-adapted trees and an expansion of cold-tolerant and drought-tolerant plants. This synthesis indicates the cooling-induced weakening of the Asian summer monsoon during the mid-Holocene. The compelling evidence from this low-latitude mountainous region strongly supports the proxy-based paradigm of a mid-Holocene cooling transition, and underscores the robust positive relationship between temperature and summer monsoon intensity, which is the key to predicting future monsoon behavior.