The Heihe River is the second largest inland river in China, and plays vital roles in sustainable societal and economic development in northwestern China. The water conservation forest, located on Qilian Mountain where the source of the Heihe River is located, provides a clean water resource. When analyzing ecohydrology, the chemical characteristics of forests are important for understanding mechanisms of water purification. Therefore, in this study, we focus on the chemical effects of forests on water quality. We selected the Tianlaochi catchment, which is located in the upper reaches of the Heihe River on Qilian Mountain, as our study area. After rainfall events, we collected samples of rainfall, through fall, stemflow, and litter seepage. Ion mass concentrations and pH values were measured for each of these components. The 12 ionic species whose mass concentrations were measured were K⁺, Ca²⁺, Na⁺, Mg²⁺, NH₄⁺, Cl^-, SO₄^2-, NO₃^-, Cu²⁺, Zn²⁺, Pb²⁺, and Cd²⁺. The results showed that the pH value of the rainfall was 7.74, and low absolute ionic mass concentrations in the precipitation were low, the most abundant ion being NO₃^- with a mass concentration of 1.1111 mg/L, and the least abundant being Na⁺ with a mass concentration of 0.0108 mg/L. With respect to that of rainfall, the pH values of through-fall from Qinghai spruce (Picea crassifolia) and Qilian juniper (Sabina przewalskii) forests decreased to 7.34 and 7.50 respectively. The P. crassifolia forest canopy could increase the mass concentration of NH₄⁺, while the S. przewalskii forest canopy showed the opposite effect. The canopies of both species could reduce NO₃^- and Cu²⁺ and increase concentrations of other ions. With respect to that of through-fall, the pH values of stemflow from P. crassifolia and S. przewalskii forests increased to 7.42 and 7.72 respectively, and anionic species increased in both types. All cationic species in the litter seepage from S. przewalskii forest decreased, but for P. crassifolia forest stemflow, the mass concentrations of Ca²⁺,K⁺,Mg²⁺, and Na⁺ decreased, and Cu²⁺ and NH₄⁺ increased. With respect to that of through-fall, the pH values of litter seepage in P. crassifolia and S. przewalskii forests increased to 7.54 and 7.59 respectively, and all anionic species in the litter seepage from both types of forest increased. In litter seepage from P. crassifolia, all cationic species decreased. In litter seepage from S. przewalskii forests, the mass concentrations of Ca²⁺, K⁺, and Mg²⁺ increased, and NH₄⁺, Na⁺, and Cu²⁺ decreased. In addition, Pb²⁺ and Cd²⁺ were not detected in all the water samples, and Zn²⁺ was only detected in the stemflow of the P. crassifolia forest.