【Objective】 With the input of acid, the lime effect of ameliorants on acidic soils is gradually weakened resulting in soil re-acidification and aluminum toxicity. As a new alternative amendment material for acid soils, the performance of biochar on alleviating aluminum toxicity during soil re-acidification is still unclear. To investigate the effect and mechanisms of biochar on alleviating aluminum toxicity during soil acidification, a comparative study between biochar and Ca(OH)₂ was conducted through a simulated soil re-acidification test.【Method】 Cyclic acid leaching with HNO₃ was used to rapidly simulate the soil acidification process. A root elongation experiment with maize was used to investigate the response of plant roots to soil acidification. The change in soil pH, soluble Al³⁺ and base cations (K⁺, Ca²⁺ and Mg²⁺) during soil acidification were also studied. 【Result】 Cyclic acid leaching effectively simulated the process of soil re-acidification. With the increase of cyclic acid leaching time, soil pH decreased and the maize root elongation was inhibited. Compared with Ca(OH)₂ treatment, biochar significantly inhibited the acidification process and alleviated adverse effects on plant roots. When acid input was simulated for 12 years, the relative elongation of maize root in biochar treatment was 18.6% higher, and the relative absorption of Evans blue was 19.6% lower than that in Ca(OH)₂ treatment. On the one hand, biochar slowed down the decrease of soil pH during re-acidification through the protonation of surface anionic functional groups, and thus inhibited the activation of soil aluminum. As a result, the soil pH and the soluble Al³⁺ concentration in biochar treatment were 0.12 units higher and 33% lower than that in Ca(OH)₂ treatment with simulated 12-year acid input, respectively. On the other hand, biochar released Mg²⁺ continuously during soil re-acidification. In the simulation of 12-year acid input, the concentration of Mg²⁺ in soil solution and uptake of Mg²⁺ by maize in biochar treatment was more than twice higher than that in Ca(OH)₂ treatment. A higher concentration of Mg²⁺ can help alleviate the symptoms of aluminum toxicity in maize by regulating the physiological response of plants to Al³⁺. 【Conclusion】 Compared with Ca(OH)₂, biochar presented more long-term potential in ameliorating acidic soils under continuous acid input. These results are of important significance for management of soil acidification.