Based on comparison between the effects of NO₃^-, Cl^- and SO₄^2- on the adsorption of Cu²⁺ by variable charge and constant charge soils, the mechanism of the anion effect was studied. Results show that when the initial concentrations of NO₃^-, Cl^- and SO₄^2- in the systems studied were identical, the adsorption percentage of Cu²⁺ by Hyper-Rhodic Ferralsol was larger in the SO₄^2- system than in the NO₃^- and Cl^- systems, while the adsorption percentage of Cu²⁺ by Cambisol in the three anion systems were very close to each other. The adsorption percentage of Cu²⁺ by Rhodic Ferralsol in the systems containing different concentrations of NaCl differed insignificantly, suggesting the effect of ionic strength on the adsorption of Cu²⁺ by variable charge soils was small. The adsorption percentage of Cu²⁺ by Hyper-Rhodic Ferralsol in the systems containing NO₃^-, Cl^- and SO₄^2- increased with increasing pH of the media. At the same pH value, the percentage in the system containing NO₃^- was close to that in the system containing Cl^-, but lower than that in the system containing SO₄^2-. The adsorption percentage of Cu²⁺ by Hyper-Rhodic Ferralsol and Rhodic Ferralsol decreased with increasing concentration of Na₂SO₄. However, the percentages were larger in the systems containing 0.005 mol L^-1 and 0.05 mol L^-1 Na₂SO₄, but smaller in the system containing 0.5 mol L^-1 Na₂SO₄ than in the system without Na₂SO₄. On the other hand, all the adsorption percentages of Cu²⁺ by Cambisol were smaller in the systems containing different concentrations of Na₂SO₄ than in the system without Na₂SO₄. It is, therefore, concluded that kind of anion species can affect the adsorption of Cu²⁺ by soils through changing the ionic strength of the media, the degree of formation of ion-pairs and the specific adsorption of anions by soils. The mechanism of anion effect on the adsorption of Cu²⁺ is more complicated in variable charge soils than in constant charge soils.