In this study, 4 types of red soils and a lateritic red soil collected from Anhui, Hunan, Zhejiang and Guangdong were used in an indoor incubation experiment to investigate effects of bio-ash ameliorating these soils in acidity. The bio-ash was produced by the burning of poplar skin mixed with rice and wheat straws in a power plant. Results show that application of bio-ash increased soil pH, decreased soil exchangeable Al³⁺. The ameliorating effect of 8 g kg^-1 bio-ash on soil acidity was better than that of 4 g kg^-1 bio-ash, and the effects were more pronounced when soil cation exchange capacity (CEC) was lower. Because the pHs of the five soils were in the range from 4.2 to 5.0 in which soil acidity was buffered by cation exchange reactions, therefore CECs of soils had important influences on ameliorating effect of the bio-ash on soil acidity. The soil with the higher CEC had the greater buffering capacity to acid, thus the pH of the soil was more difficult to be increased. The increase in pH for different soils due to the incorporation of the bio-ash followed the order: the red soil from crop land in Zhejiang +, Ca²⁺ and Mg²⁺ in these acid soils were also significantly increased due to the incorporation of the bio-ash. The soil exchangeable K⁺was increased by 0.40~0.63 cmol kg^-1, the exchangeable Ca²⁺ was increased by 1.93~2.57 cmol kg^-1, and the exchangeable Mg²⁺was increased by 0.30~0.36 cmol kg^-1. Therefore, the effect was the highest with soil exchangeable Ca²⁺ and then soil exchangeable K⁺. The content of available phosphorus also increased in these soils, particular in soils high in P content. Although bio-ash contains a certain amount of heavy metals, the application of bio-ash would not have much impact on contents of available heavy metals in the soil because of its low application rate. Therefore, the environmental risk of bio-ash application in acidic red soils is small. To sum up, application of bio-ash will not only effectively ameliorate red soil in acidity, but also improve red soil in fertility.