Taking the enclosed lake in Ningxia as the research object, the water pollution characteristics of the lake during the algal bloom in summer were continuously monitored, furthermore, the characteristics and influencing factors of water environment quality in the process of algal bloom were studied. The monitoring results showed that the main fluctuating pollutants in the process of algal bloom were CODCr, CODMn and chlorophyll, and the water quality index (WQI) ranged from 29.4 to 36.2, which was in a good grade. The correlation analysis showed that the monitoring concentration of chlorophyll a had positive correlation with WQI (R = 0.5781, P < 0.05), and the correlation of 11 monitoring indexes in 9 different regions was 15% ~ 44%, most monitoring indexes decrease with the increase of water temperature, while the concentration of chlorophyll changes with the change of CODMn, CODCr and DO, during the algal bloom process, 60.9% ~ 84.7% of the water pollution in the lake mainly came from two sources, among which TN, TP and F probably came from the recharge of the Yellow River, however, CODMn, CODCr, NH3–N and chlorophyll a may come from the process of algal bloom, and 15.3% ~ 39.1% of the pollution source is uncertain.