According to the 53 pore water samples of Beichuan River in Qinghai Province collected in 2020, the hydrochemical characteristics of the pore water in the Beichuan River Basin and the quantitative analysis of the contribution of the sources were investigated.Methods:Using ArcGIS10.7, SPSS20.0 and Origin2022 software, the hydrochemical characteristics of the pore water in the Beichuan River Basin and the quantitative analysis of the source contribution were studied based on the APCS-MLR model. Results:The results showed that:(1)The pH of the pore water in the study area ranged from 7.0 to 8.0 and was neutral to weakly alkaline. Along the direction of water flow, the mass concentrations of Ca2+ and HCO-3 gradually decreased, and the mass concentrations of Na+, Cl- and SO42- gradually increased, and the hydrochemical type was gradually transformed from HCO-3-Ca2+ and HCO-3·SO2-4-Ca2+ to HCO-3·SO2-4-·Cl--Na+·Ca2+and HCO-3·SO2-4·Cl--Na+ type, compared with the upstream, the mass concentration of TDS and major ions such as Na+, Ca2+, Cl-, SO2-4 increased significantly, and the downstream water quality became worse.(2)The hydrochemical characteristics of pore water in Beichuan River Basin were generally controlled by mineral dissolution and filtration, silicate rock, carbonate rock mineral dissolution were the main source of chemical components of groundwater, while cation exchange was strong, upstream to downstream, evaporation and concentration increased significantly, and the increase in human activities, which had a certain impact on the pore water hydrochemical characteristics.(3)The pore water in the upstream of Beichuan River Basin was mainly controlled by hydrogeochemical processes in the natural state, and the APCS-MLR model showed that the contributions of carbonate rock mineral leaching, rock salt potassium salt dissolution and ion exchange, and evapotranspiration and concentration were 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.Conclusions:The activity of industry and agriculture in the downstream pore water was greater than that in the upstream.We should strengthen the protection of water resources in the downstream basin.