Abstract: The Yamzhog Yumco watershed is the key region of ecological environment protection in China, but pH value in Yamzhog Yumco is often too high, which puts the local government under tremendous pressure. In order to understand the reasons for the high pH in Yamzhog Yumco, geochemical methods were used to find the clues to its formation mechanism based on the hydrochemistry and weathering in the watershed. The results show that: (1) pH value of Yamzhog Yumco is between 8.5 and 9.3, with the exceeding probability of 49% and seasonal variation, while median value of pH in water sources from the catchment (including surface water, groundwater and glacier melt water, etc) is 8.52, representing the background level of pH in the watershed. (2) Hydrochemical type of water source in the Yamzhog Yumco Basin was Ca-HCO₃ or Ca-SO₄, but it was changed to Mg-SO₄ type after a long period of evaporation and complicated geochemical processes. (3) The weathering in the Yamzhog Yumco Basin is not only characterized by the weathering of carbonate and silicate rocks, but also by the weathering of sulfide minerals. The high pH value in Yamzhog Yumco is the result of watershed weathering and long-term natural evolution of Yamzhog Yumco, and it is not affected by with the water sources. The abundant HCO₃^- provided by the watershed weathering is the basis for pH evolution in Yamzhog Yumco under specific conditions. Retention and accumulation of HCO₃^- in Yamzhog Yumco caused by the closed hydrodynamics is the prerequisite for pH evolution in Yamzhog Yumco. The partial conversion of HCO₃^- to CO₃^2- caused by long-term evaporation leads to an increase in the water pH in Yamzhog Yumco. In short, the high pH of Yamzhog Yumco is the result of the watershed weathering, the closed hydrodynamics and the long-term evaporation of the water body. Since the high pH value of Yamzhog Yumco is the result of long-term natural evolution, it is recommended to remove pH from the water quality management list to avoid unnecessary management costs and pressure.