We determined the concentrations of major ions (Ca²⁺, Mg²⁺, Na⁺, K⁺, and HCO₃^-, SO₄^2-, and Cl^-) in the waters of Nenjiang River system, China, analyzed the relative contributions of chemical weathering of different kinds of rocks, atmospheric input, and human input to the total dissolved solutes, and estimated the chemical weathering rates of different kinds of rocks and associated atmospheric CO₂ consumption rates in the whole basin and each subbasin. The water of the Nenjiang River system was weakly alkaline, with an average pH of 7.5, calcium was the dominant cation, accounting for 50% of the total cations, and HCO₃^- was the most abundant anion, accounting for 85% of the total anions. The dissolved cations in the waters were mainly originated from silicate weathering (～38%) and carbonate weathering (～32%), and from the dissolution of evaporates (～25%), human activities (～5%), and atmospheric precipitation input (<1%). The average chemical weathering rate of silicate rock in the basin was approximately 1.37 t·km^-2·a^-1 (total dissolved solids of silicate rock), and the associated atmospheric CO₂ consumption rate was about 40.1×10³ mol·km^-2·a^-1.