Abstract: In recent years, the Cladophora bloom in the nearshore area of Qinghai Lake is becoming more and more serious. Based on field investigation, data collection and remote sensing image interpretation, the changes of water quality and distribution characteristics of Cladophora bloom in Qinghai Lake were analyzed. The results showed that the major water quality parameters of Qinghai Lake remained good from 2011 to 2019. Although there were some annual variations, no significant deterioration trend of water quality was observed. In 2019, the field survey found that there was a considerable amount of Cladophora in the coastal shallow water area of Qinghai Lake, and the biomass of Cladophora was the largest in the area around Bird Island and Buha River estuary, averaging 5213.4 g/m² in August. Remote sensing image analysis showed that Cladophora was mainly distributed in the west and northwest coast of Qinghai Lake and near the estuary of the lake spatially, and the coverage area of Cladophora decreased first and then increased from 1987 to 2019. The analysis showed that the increase of water area suitable for the growth of Cladophora due to the rise of water level in Qinghai Lake in recent years may be the main reason for the expansion of Cladophora bloom. At present, the impact of Cladophora bloom on the water ecological environment of Qinghai Lake is still not clear, and further attention should be focused on the water ecosystem health of Qinghai Lake under the impact of both climate change and anthropogenic activities.