Abstract: In order to study the hydrochemistry formation mechanism of mountainous rivers in Xiaoqinling, the Taiyu reservoir in Tongguan area has been taken as a research example, the hydrogeological condition investigation has been carried out and the test data has been analyzed. The results show that the chemical components of upper valley reservoir water in recharge area is not generally increases, but its salinity increases, with the hydrochemical types of HCO₃·SO₄-Ca and Mg. And the hydrogeochemical evolution process is based on the two aspects of chemical reaction and evaporation, which is dominated by the chemical reaction. ①The mineral saturation indexes of all sample points have been calculated by Phreeqci method, while the water mass balance has been simulated by the Netpath method. And then, it is concluded that the reservoir water quality happened Ca/Na exchange and mineral dissolution and precipitation. And the water chemistry includes the dissolution of halite, gypsum, calcite and the precipitation of ultrabasic rock mineral composition. ②The quality of components has increased 0.23 times through evaporation concentration.