Xingyun Lake is located in central Yunnan Province, Southwestern China with 1722 m a.s.l. Qinghai Lake is situated in southwestern Yunnan Province with 1885 m a.s.l. Lugu Lake, the third largest deepwater lake in China, is located on the boundary between Yunnan and Sichuan provinces with 2690 m a.s.l. Wuxu Lake is located in southwestern Sichuan Province,the eastern branch of the Hengduan Mountains on the southeastern margin of the Qinghai-Tibetan Plateau and its altitude is 3760 m a. s. 1. These four lakes are located in different latitude and altitude regions in Southwestern China, and they represent different topographic, climatic and vegetation units. Four continuous sedimentary cores were recovered using a UWITEC piston corer from Xingyun,Qinghai,Lugu,and Wuxu lakes, respectively. The geographical location and water depth of these four cores are 24°21 '33.4"N, 102°47'11.1"E, 8.5 m in depth; 25°7'56.8"N, 98°34'19.2"E, 6.3 m in depth; 27°43'08.4"N, 100°46'33.9"E,69.3 m in depth; and 29°9'11.48〃N,101°24'21.6"E,30 m in depth, respectively. 127 pollen samples from Xingyun Lake,377 pollen samples and 600 diatom samples from Lugu Lake,278 pollen samples from Qinghai Lake, and 276 pollen samples from Wuxu Lake were analyzed, respectively. These pollen and diatom records spanning ca. 20 ka (1 ka = 1000 cal. a B.P.) reveal histories of climate change and the South Asian monsoon since 20 ka in Southwestern China. The results show that the initial Late Glacial warming in Southwestern China was at 19.0 ~18.0 ka. After the initial Late Glacial warming, the climate was different in different regions of Southwestern China. Namely, the Heinrich Event 1,the Bolling-Allerod warm period and the Younger Dryas event during the last deglaciation are ubiquitous in the west and south regions of Southwestern China, affected solely by the South Asian monsoon. The start of the Holocene is recorded at 11.5 ka. Since then, the temperature and humidity began to slowly increase. From ca. 10.0 ka, the climate warmed and humidified faster, and culminated at the mid-Holocene. Then, the climate deteriorated, accompanied by evidence for human impact. However, the east and north regions of Southwestern China are simultaneously affected by the East Asian monsoon,resulting in a more complicate climate effect. These climate changes since 20 ka in Southwestern China reveal that the South Asian monsoon began to strengthen gradually at ca. 19.0 ka. In the course of the gradual strengthening of the monsoon, there are two obvious monsoon weakening periods, corresponding HI and YD events, respectively. The South Asian monsoon strengthened further at 11.5 ka and reached to the maximum at the mid-Holocene, and then the monsoon gradually weakened. Based on this study, we consider that the initial late glacial warming and the tendency of gradual increase of temperature are controlled by summer solar insolation. At the same time, the variations in greenhouse gas concentrations also modulate the overall structure. Whereas the Heinrich Event 1 and the Younger Dryas event may be related to the slowdown of the ocean's thermohaline circulation. Holocene climatic optimum occurred at the mid-Holocene in the South Asian monsoon regions, which was likely caused by the interplays of multiple large-scale boundary conditions,including direct and indirect insolation controls,sea-level change,sea-surface temperature, and remnant ice sheets.