Lake geomorphology and high-level lacustrine deposits since the mid-late Pleistocene are well preserved in lakes of the Qinghai-Tibet Plateau. According to geological surveys of 17 lake districts in different locations of the plateau,combined with interpretations of satellite images and topographic maps, the authors studied the timing of formation and scopes of the pan-lake areas of the plateau and their paleoclimate. The latest two high lake levels (overflow surfaces) on the Qinghai-Tibet Plateau in the Quaternary occurred at ～40 to 30/35 ka and ～65 to 53 ka respectively. In these time intervals, the plateau was covered by huge interconnected pan-lake systems with a total area of ～36 km2 and a total volume of lake water of ＞530 million km2, which are about 38 times and 659 times larger than those of the modem lakes respectively. Before this pan-lake period in the late Pleistocene, there had been three high lake levels that occurred at ～132-112 ka, 110-95 ka and 91-72/～83-75 ka respectively, suggesting that the late Quaternary climate on the plateau was unstable and changed rapidly. The ～40-30 ka high lake level also appeared in the Tengger desert north of the plateau,suggesting that there existed very strong summer monsoons from South Asia then; the variation in solar radiation with a 20,000 precessional period has special importance for the high-altitude Qinghai-Tibet Plateau in the low-latitude zone of the Earth. Around 30 ka, the pan-lakes at the peripheries of the Qinghai-Tibet Plateau drained out suddenly with rapid uplift of the plateau and cooling. In a short time the huge amount of cold lake water emptied into the Indian Ocean and Western Pacific. The draining event of the pan-lakes brought about the environmental changes of rivers and lakes at peripheries of the plateau.