The water areas of Lake Qinghai in the past twenty years have been identified using NOAA/AVHRR satellite data. Theresults show that the area of Lake Qinghai has been decreased in these years, and the area of decreasing is mainly in the north of LakeQinghai. To find out the climate elements which affect the area of Lake Qinghai, precipitation, temperature and evaporation in theregion of Lake Qinghai are analyzed. The results show that increasing in temperature and evaporation, and the decreasing inprecipitation are the main reasons for the decreasing in the area of Lake Qinghai.