Developing a scientific understanding of hydrochemical characteristics and their controlling factors is of great significance in protecting the quality of the water environment and maintaining ecological balance. The article presents the water chemical characteristics and their controlling factors in the Xiying River Basin (XRB) during different flood seasons from April to November, 2015. The results are as follows. The river water was found to be slightly alkaline on the whole and the dominant ions were Ca²⁺ and HCO₃^-. The hydrochemical type was Ca²⁺-Mg²⁺-HCO₃^-. Besides that, the ion concentrations showed great differences in different periods. From the pre-flood period (PreFP) to post-flood period (PoFP), total dissolved solids (TDS), Ca²⁺, Mg²⁺, Na⁺ and SO₄ ^2- presented a trend of decreasing first and then increasing. TDS reached its maximum in PreFP. K⁺, NO₃^- and HCO₃ ^- presented a trend of increasing first and then decreasing, reaching a maximum in SuFP. Rock weathering is the major controlling mechanism of river hydrochemistry, of which carbonate weathering is the main source of ions, followed by silicate weathering. At the same time, factors such as precipitation, river supply sources, human activities and vegetation cover in different times all have significant influences on river hydrochemistry. The findings in this paper show that we need to adjust cultivated land areas and their space distribution scientifically, strengthen the construction of riverbank forest belts, and make full use of the effects of forest land and grassland. These measures could improve water quality effectively and guarantee water security in middle and lower reaches.