The nitrogen and oxygen isotopic composition and hydrochemical characteristics of water were studied systematically to determine the main sources of nitrate in Lake Caohai and its rivers, and the contributions of each source were quantitatively analyzed. The ratio of NO₃^-/Cl^- to Cl^- in the lake water showed that the lake was mainly affected by livestock manure and urban sewage in wet season, while the ratio in rivers and wells indicated that rivers were affected by agricultural activities and urban sewage. The analysis of δD-water and δ¹⁸O-water showed that atmospheric precipitation was the main source of water in Lake Caohai and its rivers, with a strong evaporation. The value of δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- in lake water were -5.56‰-11.30‰ and 0.02‰-25.40‰ respectively, which were more negative than the rivers and more positive than the wells. The results of SIAR model showed that nitrate in Lake Caohai and its rivers mainly came from agricultural activities related to fertilizer, soil organic N and livestock manure, and its contributions was more than 60%. The contribution rate of urban sewage was about 25%. The contribution of atmospheric precipitation was mainly reflected in the lake water.