To identify the sources of nitrate in irrigation canal water bodies of the Ganfu Plain, pH, electrical conductivity, concentrations of dissolved oxygen and NO-3, and nitrate isotope compositions ( δ 15N-NO^-3 and δ 18O-NO^-3) in the West Main Canal of the Ganfu Plain during spring and autumn were analyzed. The contributions of the main pollution sources were quantitatively analyzed using the stable isotope analysis in R model based on Bayesian mixing (MixSIAR model). The results show that the nitrification in water bodies of the West Main Canal is slightly stronger in autumn than in spring but is overall weak in the two seasons, and denitrification is almost not observed. The sources of NO^-3 in water bodies of the West Main Canal mainly include the upstream water, ammonium-nitrogen fertilizer, soil nitrogen, domestic sewage and manure, as well as precipitation. The upstream water (with a contribution rate of greater than 50%) and human activities (including the ammonium-nitrogen fertilizer, domestic sewage, and manure, with a contribution rate of greater than 37%) play a dominant role in the NO^-3 concentration in the entire irrigation canal water body. The contribution rates of pollution sources to NO-3 in the irrigation canal water body in descending order are as follows: upstream water, ammonium-nitrogen fertilizer, domestic sewage and manure, soil nitrogen, precipitation in autumn, and ammonium-nitrogen fertilizer, upstream water, domestic sewage and manure, soil nitrogen, precipitation in spring. In addition, the contribution of NO^-3 sources is associated with land use patterns. The contribution rates of upstream water and domestic sewage and manure show upward trends from rural to urban areas, while the contribution rate of ammonium-nitrogen fertilizer shows an opposite change.