Water chemical composition of a lake and its inflow rivers is an important lake characteristic, which can reflect the climatic and environmental background of the basin where the lake is located and the river flows through. Water samples were collected and their major ions were measured from the major lakes (i. e., Yamzhog Yumco, Bajiu Co, Chen Co, Kongmu Co and Pumoyong Co), inflow rivers, and wells in the Yamzhog Yumco Basin. Possible controlling factors on the major ion compositions from different waterbody were analyzed. Results revealed that the major ion compositions (e. g. hydrochemical types) of the water in the basin were significantly different as follows. The major ion compositions of lake water in Yamzhog Yumco, Bajiu Co, Chen Co, Pumoyong Co and Kongmu Co were SO₄^2--HCO₃^--Mg²⁺-Na⁺, SO₄^2--Mg²⁺-Na⁺, SO₄^2--Na⁺-Mg²⁺-Ca²⁺, HCO₃^--SO₄^2--Mg²⁺-Ca²⁺, and HCO₃^--SO₄^2--Ca²⁺, respectively. For the inflow river water, the major anions were HCO₃^- and SO₄^2- and the preponderant cation was Ca²⁺. Comparing with the water chemical compositions of inflow rivers, the contents of Mg²⁺, Na⁺ and SO₄^2- in Yamzhog Yumco and Chen Co were much higher with a relative low contents of Ca²⁺ and HCO₃^-. On the contrary, there was little difference for the Kongmu Co and its inflow rivers. In addition, the major ion compositions of well waters were characterized by HCO₃^--Ca²⁺. Further analysis indicated that the water chemical compositions were mainly controlled by rock weathering in the whole basin, and also by evaporation and crystallization for Yamzhog Yumco, Bajiu Co and Chen Co. For the inflow river waters in the basin, the main controlling factors of chemical compositions were carbonate weathering and evaporite weathering, while the effect of the silicate weathering on the river water chemistry was minor. Thereinto, the dominant controlling processes were carbonate weathering for inflow river waters flowing into Yamzhog Yumco, and the evaporite weathering for the inflow river waters flowing into Chen Co and Kongmu Co. As for the different water chemical characteristics between the inflow river water and lake water of Yamzhog Yumco and Chen Co, the possible reason should own to the strong evaporation of two lakes which may intensify calcium phosphate precipitation. The small lake area of Kongmu Co and the vast amounts of inflowing river water can explain the little difference of water chemical compositions between the lake and its inflow rivers. The study also showed that the controlling processes of water chemical compositions among inflow rivers were different because the different rock-stratigraphic area they flowed was dominated by physical weathering/erosion. With global warming, the style and intensity of rock weathering would be changed. So it is indispensable to further investigate the speed of rock weathering in the basin and its possible response to climate change.