Water temperature, pH, EC, TDS, ORP, Si and major anions (Cl-, NO32 and SO42-) and major cations (Na+, K+, Ca2+ and Mg2+) in the Danjiangkou Reservoir, the water source area of the Middle Route of the South to North Water Transfer Project of China were monitored during the period of 2004-2006. Analysis of variance (ANOVA) and principal component analysis (PCA) were performed to explore their seasonal variations and origins. The results show that the water is slightly alkaline with an average pH ranging from 7.9 to 8.4. The water is HCO3--Ca type with low mineralized degree, and its total dissolved solid ranges from 174.6-209.1 mg·L-1. The major ions range are as follows: Cl-, from (4.0±0.5) mg·L-1 to (6.9± 1.8) mg·L-1; NO3-, from (4.6 ± 0.9) mg·L-1 to (6.8±1.7) mg·L-1; SO42-, from (24.3±2.7) mg·L-1 to (35.4±6.9) mg·L-1; HCO3-, from (133.0±11.7) mg·L-1 to (153.5±29.6) mg·L-1; Na+, from (2.0±0.3) mg·L-1 to (5.3±1.0) mg·L-1; K+, from (0.7±0.09) mg·L-1 to (1.6±0.7) mg·L-1; Ca2+, from (33.0±2.1) mg·L-1 to (46.6±0.8) mg·L-1; Mg2+, from (8.0±2.5) mg·L-1 to (10.5±3.2) mg·L-1. Statistical analyses indicate that the water quality variables display significant seasonal differences except HCO3 and Si. In general, major ion concentrations in flood season are relatively lower due to the dilution of precipitation. HCO3- accounts for 75%-88% of the total major anions, while Ca2+ and alkali-earth metals (Ca2++Mg 2+) account for 60%-80% and 87%-96% of the total major cations respectively, reflecting that carbonic acid weathering is the main proton producer in the water. The major ions have no adverse effects on human according to water drinking quality guidelines of China and WHO.