Abstract: The biological toxicity of Cd²⁺ and Cu²⁺ to Carassius auratus, Pseudorasbora parva and Daphnia magna was studied under different water hardness. The results revealed that at the hardness of 80 mg/L CaCO₃, the median lethal concentration (LC₅₀) of Cu²⁺ to P. parva and D. magna was 100 and 79 μg/L, respectively; and at the hardness of 250 mg/L the LC₅₀ of Cu²⁺ to the two organisms was 160 and 159 μg /L, respectively. The LC₅₀ of Cd²⁺ to C. auratus at the hardness of 50 and 250 mg/L was 3.69 and 16.3 mg/L, respectively. The results showed that the acute toxicity of Cu²⁺ and Cd²⁺ to aquatic organism decreased with the increase of water hardness. The hardness slope of toxicity of Cu²⁺ and Cd²⁺ was 0.513 and 0.923, respectively, and the influence of hardness on toxicity of Cd²⁺ was greater than that of Cu²⁺.The results of subacute toxicity test for C. auratus showed that the chronic toxicity and quantity of Cd²⁺ absorbed by the fish were reduced when the hardness increased. It revealed that the higher water hardness was able to reduce the bioavailability of heavy metal.