Abstract: According to the monitoring data of total nitrogen in Erhai Lake,the temporal and spatial variation trend of the concentration of total nitrogen from 1985 to 2019 was analyzed. The results showed that the annual average total nitrogen of the lake indicated a rising trend in fluctuation,and reached a historical peak in 2006. The spatial distribution of total nitrogen concentration showed that the concentration in the northern area was higher,followed by the south,and the concentration in the middle is slightly lower. In the same section,the concentration in the nearshore point was higher than that in the center of the lake. The monthly mean value of total nitrogen for the year of 2002—2019 showed obvious seasonal change rule,the maximum value was found in August and September,and the minimum value appeared in March and April. The annual average value of the seven main rivers entering Erhai Lake was higher than that of Erhai Lake,which affected the water quality of Erhai Lake.