Selecting the upper rea ch of the Chishui River as study object, its spatial variation of hydro -chemical characteristic andsources of major ions （especially SO42-）were analyzed. The results show that the Chishui River belonged to low salinity and hard water inwater quality. HCO3-and Ca 2+are the dominant anion and cation, and the Chishui River is a typical carbonate river. Bordered by theBaisha River, the hydro -chemical composition of the upper river changed in smooth, major ions came mainly from the input of naturalenvironment. Because of the effect of human factors, the volatility of hydro-chemical composition has appeared in the lower river. SO42-isthe secondary anion, accounting for 30% of the total anion. The correlation of SO42-and Ca2+, HCO3-and Mg2+are good, while those of HCO3-and Ca2+are poor. So, we inferred that the SO42-is from the dissolution of gypsum. However, the dissolution of gypsum can not fully explainthe source of SO42-, acidification is also an important way affecting the content of SO42-in the river.