The investigation of water pollution from mainstream and dominating tributaries at different Hydrological conditions of the Qiantang River, East China, showed that water quality tends to deteriorate from upstream to downstream and possessed severe pollution levels in the dry season rather than the abundant and medium seasons. Analysis of PCA indicated that TN, NO¯₃, NH_3¯N, and TP with maximum values of 18.70, 2.57, 15.50, and 0.80 mg·L^-1 were the dominant pollution factors, therein NO¯₃ accounted for the largest percentage of TN (up to 83.1% in various nitrogen species). Domestic sewage and the discharge of poultry and animal feces were the main point source pollution, and fertilizer that flushed into the river leaching from farms possessed the maximum proportion of non-point source pollution associated with NH_3¯N and TP. P mainly came from sewage discharge simultaneously. In addition, the Dayuan River was the tributary with the most severe pollution and owned significantly higher TN and NH_3¯N concentrations than that in mainstream and the medium values, reaching up to 3.17 and 8.99 mg·L^-1. Low DO concentration should be an important cause for denitrification from NO¯₃ to NO¯₂ and NH_3¯N in the Dayuan River.