Qinghai Lake is an important water body that maintains the ecological integrity of the northeastern Qinghai-Tibet Plateau and is also a natural barrier to control the spread of the western desertification to the east. And the study of the changes Qinghai Lake area is of great significance to regional ecological security and economic development. Therefore， this study aims to investigate the trends and causes of the changes of Qinghai Lake area from 1990 to 2020. With the support of Google Earth Engine （GEE） platform， Landsat data were used to monitor the dynamic changes of Qinghai Lake for winter-spring and summer-autumn in the past 30a， and the reasons for the changes were analyzed in combination with meteorological and hydrologic data. The results show that：（1） With 2004 as the inflection point， the area of Qinghai Lake showed a trend of decreasing first and then increasing during 1990 to 2020. In recent years， the area increase is most obvious. During 1990~2004， the area of Qinghai Lake decreased by 140.36 km² in winter-spring， about 3.19% of the area in 1990； and decreased by 157.547 km² in summer-autumn， about 3.56% of the area in 1990. During the period 2004~2020， the area of Qinghai Lake increased by 336.59 km² in winter-spring， about 7.89% of the area in 2004； and increased by 349.3814 km² in summer-autumn， about 8.18% of the area in 2004. （2） In terms of intra-annual variation， the change in area from winter-spring to summer-autumn was large from 1990 to 2000， with an average change value of 26.09 km²/a， and the intra-annual variation was small after 2000， with an average change value of 5.87 km²/a. （3） From 1990 to 2004， the instability of precipitation and runoff and high steady evaporation in Qinghai Lake area were the important reasons affecting the area of Qinghai Lake. High evaporation， low precipitation and low runoff together led to the decline of the area of Qinghai Lake. From 2004 to 2020， the simultaneous increase of precipitation and runoff makes the area of Qinghai Lake increase significantly， assisted by the weak decreasing trend of evapotranspiration. （4） The shoreline morphology of Qinghai Lake is constantly changing， and the most obvious changes are in Shadao Island on the east bank， Niaodao Island and Tiebuka Bay on the west bank， and the entrance of Shaliu River on the north bank. It may be caused by the sediment of runoff soil and the dynamic mechanism of lake water. The specific reasons need to be further discussed. This study has certain value for the study of lake hydrology and the response of lakes to climate change at different scales.