Lake Qinghai lies on the northeast corner of the Tibet-Qinghai Plateau. It is a closed-basin lake and the largest water body in China with an area of about 4437 km2. Aragonite and calcite are precipitating from the brackish (TDS 12-14 g/l) and alkaline water (pH 9.1-9.4). Summer rainfall exerts an important control on changes in both lake level and water chemistry. As the lake today is situated at the outer margin of the Asian summer monsoon, past climate changes were sensitively documented in the sedimentary record of the lake. This thesis reports new results obtained after 1991 from two high quality cores, Q14B and Q16C, and on the 26 m drill core. It aims to provide key-site information for the international effort and database of the IGBP-PAGES community on regional paleoenvironmental and paleoclimate changes since the Marine Isotopic Stage 3. The study established a detailed carbonate mineral stratigraphy for the postglacial calcareous succession of the lake. The postglacial carbonate deposition underwent five main phases, each of them representing a distinguishable P-E balance period of the paleo-lake. The study provided new data showing that the isotopic ratios of some primary carbonates such as aragonite laminae do not fall into the d13C-d18O covariant trend. Primary dolomite showed most negative d13C values and their isotopic ratios dissent from the covariant trend. Nevertheless, neither the isotopic ratios of aragonite laminae nor those of primary dolomite represent a breakdown of the d13C-d18O covariant trend, the essential isotopic identity of the closed-basin lake. Results from the seismic Profile 1 and the 26 m drill core from the eastern basin reveal that Lake Qinghai expanded rapidly around 68.7 ka 14C BP and remained in wet but not in full glacial conditions until about 28.8 ka 14C BP. The largest extent of the paleo-lake size was much smaller than that of the Holocene. The sub-bottom sediment formed during this period of time show an offlap sequence, clearly indicating a general trend of shrinking in lake size towards the Last Glacial Maximum (LGM). The lake was in severely cold and arid conditions at the LGM (~28.8-18.3 ka 14C BP), as suggested by the windblown loess-like sandy deposits and the seismic data. The 26 m drill core at the central eastern basin did not reveal moraine deposits, suggesting an exclusion of any glacial advance down to the central areas of the lake during both MIS 3 and MIS 2. Climatic conditions during the MIS 3 were warmer than during the MIS 2 but definitely colder than during the Holocene. The MIS 3 wetter climate therefore was comparatively more favourable for regional glacier advance rather than the MIS 2 in the northern Tibet-Qinghai Plateau. The climate before 11.6 ka 14C BP was much colder and drier than during the Holocene, as evidenced by a shallow lake environment with lower carbonate production and lower organic productivity. The seasonal inflow of sediment-laden water increased abruptly from ~11.6 ka 14C BP, signaling an enhancement of precipitation in the large catchment. Between ~10.7 and 10 ka 14C BP, a negative water balance persisted, as indicated by a development of a carbonate playa lake.