Abstract: Aimed at providing a reference for selecting suitable amendment for southern acid paddy soil, this study explored the effect of biochar in three soils with soil acidity of pH5.0, pH5.5, and pH6.0. This study explored the passivation remediation effect and possible mechanism of biochar on heavy metal Cu and Cd contaminated soil under different acidity conditions. An equilibrium desorption experiment and incubation batch experiments were used to study the addition of 2% bichar on the effects of three acidity soils on soil pH, copper and cadmium retention, and copper and cadmium states. An equilibrium desorption experiment showed that the retention of copper and cadmium by biochar at pH 5.0 and pH 5.5 can reach 76.78%, 36.77%, 65.95%, 70.62%, respectively. The research results of soil cultivation for 30 days show that biochar can increase soil pH by 0.48~0.71 units; the improved Tessier five-step continuous extraction method is used to extract Cu and Cd states in the soil. The addition of biochar transforms copper from easily reducible iron-manganese-bound state to residue state and cadmium from exchangeable and carbonate-bound states to crystalline iron oxide bound state.