Paleoenvironmental studies are essential for understanding the future climate changes and their impacts on our environment. A key area for investigating long-term environment changes is the Tibetan Plateau (TP) because the environmental changes of TP are highly sensitive and strong effect to climate changes. The Gonghe basin is an intermontane basin on the northeastern TP,which is influenced by Asian monsoon and westerly. Due to its unique topography, the past palaeoclimate studies in this region show a complicate humidity pattern during the Mid-Late Holocene. Genggahai Lake (36°10'50"N?36°11'57"N,100°05'33"?100°07'33"E; 2858m a.s.l.), a shallow grass-type lake in the Gonghe basin, includes Upper Genggahai Lake and Lower Genggahai Lake. The Lower Genggahai Lake is nearly desiccated,connecting with Lower Genggahai Lake through groundwater. The Upper Genggahai Lake has a surface area of about 2km~2. A 782-cm-long drilling core (GGHA) and a 750-cm-long drilling core (GGHB) were obtained from the center of Genggahai Lake (36°1 1'26"N,100°06'15"E). By comparing their grain-size and magnetic susceptibility profiles, the missing sediments from core GGHA were supplement by corresponding sediments in core GGHB. The 12 samples of aquatic plant macrofossil were selected for Accelerated Mass Spectrometry (AMS) '4C dating. In this study, we select a total of 85 samples from the upper part of the core GGHA range from 0 to 581m,which were performed for pollen analysis to detect vegetation compositions and climate changes on Gonghe basin since 6.3cal. ka B. P. The pollen data from Genggahai Lake suggested that the study area was mainly covered by desert steppe/steppe vegetation dominated by Artemisia, Amaranthaceae (Chenopodiaceae) and Poaceae over the last 6.3cal.ka. (1) From 6.3cal.ka B.P. to 5.6cal.ka B.P., the montane forest,surrounded the Gonghe basin,was composed by conifer (Pinus). Slightly high percentage of Cyperaceae pollen possibly reflects a small area of wetland around the lake. In addition, a low A/C ratio indicated a low relative humidity during this period. (2) The montane forest decline started at 5.6cal. ka B.P. and disappeared at 4.lcal.ka B.P., accompanied by expanding Artemisia-dominaled steppe/desert steppe and shrinking Cyperaceae-dominated swamp meadow. The increasing value of A/C ratio indicated that the climate inside the Gonghe basin became relatively humid during this period. (3) During the 4.1 ~ 3.lcal.ka B.P., the variable A/C ratio indicated highly fluctuated humidity in study region. Local vegetation was still covered by steppe and desert steppe, and the swamp meadow degraded because of limited moisture. (4) After about 3. lcal.ka B.P., the deceasing percentages of Cyperaceae and Artemisia and the increasing percentage of Ephedra and Nitraria (typical arid plants) illustrated that the climate was more arid than former stage. The vegetation surrounding Genggahai Lake was mainly desert steppe, and the area of steppe, dominated by Artemisia and Poaceae, was decreasing. Generally,our research has an important implication for understanding the long-term vegetation history responses to climate forcing on the northeastern TP. The pattern of vegetation changes indicates a drying climate trend, consistent with other independent climate records in relation to a weakening summer monsoon and decreasing summer insolation during the Mid-late Holocene. And, the regional humidity is responsible for forest decline during this period.