Based on 53 groundwater samples collected from Beichuan River in Qinghai Province in 2020, the hydrochemical characteristics of the groundwater in the valley plain of Beichuan River Basin and the quantitative analysis of the contribution of the sources were investigated based on the APCS-MLR model by using the software of ArcGIS10.7, SPSS20.0, and Origin2022, etc., and the results showed that: (1) The pH of the groundwater in the study area ranged from 7.0 to 8.0 The pH of groundwater in the study area ranges from 7.0 to 8.0, which is neutral to weakly alkaline. Along the direction of groundwater flow, the percentages of Ca2+ and HCO3- gradually decreased, and the percentages of Na+, Cl- and SO42- gradually increased, and the hydrochemical type was gradually transformed from HCO3-Ca and HCO3·SO4-Ca to HCO3·SO4·Cl-Na, Ca and HCO3·SO4-Ca. Ca and HCO3·SO4·Cl-Na type, and compared with the upstream, the content of TDS and major ions such as Na+, Ca2+, Cl-, and SO42- increased significantly, and the downstream water quality became worse. (2) The overall pore water hydrochemical characteristics of Beichuan River Basin are controlled by mineral dissolution and filtration, and the dissolution of silicate rock and carbonate rock minerals is the main source of the chemical components of the groundwater, while the cation exchange effect is strong, and the upstream to downstream, evaporation and concentration increases significantly, and the increase of human activities has a certain impact on the chemical characteristics of the groundwater. (3) The pore water in the upstream of Beichuan River basin is mainly controlled by hydrogeochemical processes in the natural state, and the APCS-MLR model shows that the contributions of carbonate rock mineral dissolution and filtration, rock salt potassium salt dissolution and ion exchange, and evaporation and concentration are 35.93%, 29.68%, and 26.54%, respectively. The contribution of silicate rock dissolution, evaporation concentration, industrial activities, agricultural activities, and carbonate rock mineral dissolution in the downstream pore water were 41.27%, 27.73%, and 22.91%, respectively, and the water protection in the downstream watershed should be strengthened.