【Objective】The purpose of this study was to explore the optimum preparation conditions of bone biochars, their improvement effects on acid soil, and the influence on Al forms from the perspective of soil solid phase and liquid phase.【Method】The anaerobic pyrolysis of pig, chicken, ox, and sheep bone meals were used as raw materials to produce biochars (bone biochars) as ameliorants and three typical acid soils were used.【Result】The results showed that the bone biochars prepared by high-temperature anaerobic pyrolysis were rich in CaO, CaCO3, and Ca5(PO4)3(OH), contained alkaline substances such as NaO and MgO and characterized by many functional groups including -OH. Nevertheless, the alkali content of bone biochar was greatly affected by the pyrolysis temperature. The alkali content was close to the peak value and remained stable at 800 ℃ for chicken and cow bones, and the same at 900 ℃ for pig and sheep bones. Interestingly, the alkali content of these bone biochars was about 90% of the quicklime, which is the best preparation temperature. Also, the biochars effectively increased the pH of acidic red soils from Anhui, Jiangxi, and Guangdong with an original pH of < 6. The alkaline substances in bone biochars mainly existed in the form of H+ buffers and the magnitude of the increase in soil pH was inversely proportional to the initial soil pH. After adding 5 g·kg-1 pig, chicken, sheep, and ox bone biochars to the red soils with pH = 4.40, the Al concentration in soil solution decreased by 33%, 34%, 47%, and 41%, respectively, compared with the control, and the content of organically bound Al in solid phase had no significant change（P > 0.05). Bone biochars increased the cation exchange capacity of acidic soil and reduced the content of soil solution Al and exchange Al by promoting the conversion of active Al to adsorbed hydroxyl Al and more stable Al in the soil solid phase.【Conclusion】Bone biochars are rich in nutrients required for plant growth, has the dual characteristics of inorganic and organic amendments, and are a high-quality material that can replace traditional lime as a soil amendment for acidic farmland.