In this study, four kinds of seafood shells were used as raw materials by anaerobic pyrolysis to produce biochars and three typical acidic soils in south China were used as test soils to explore the best preparation conditions of seafood shell biochars and their improvement effects and mechanisms on acidic soil. The results showed that the main components of seafood shell biochars prepared at high temperature were CaO and CaCO₃, and simultaneously containing alkaline substances such as NaO and MgO, meanwhile the alkali content were very high. Pyrolysis temperature had great influence on the alkali content in the biochars, 800℃ was the best temperature for preparing the biochars, at this time the alkali content reached or even exceeded lime, while retaining some of the functional groups such as -OH, which was more stable than lime, and containing less acid substances than lime. The biochars prepared at 800℃ significantly promoted acidic soil pH, solid phase adsorbed hydroxyl Al, cation exchange capacity and acid buffer capacity, decreased the contents of soil solution Al and exchange Al, and increased the contents of soil solid phase organically bound Al, which is an ideal acid soil amendment replacing traditional lime, can effectively relieve the pressure of lime resources and reduce the improvement cost. The results of this study have important guiding significance for reducing the cost of acid soil improvement and resource utilization of seafood shell waste.