The acidification characteristics of red soil and its main influencing factors under straw return was studied，providing a theoretical basis for alleviation of red soil acidification. Based on a long-term field experiment in upland red soil，the changes in soil pH，exchangeable acidity，exchangeable base cations，and organic carbon fractions index and their correlation were determined under the treatments of no fertilization(CK)，single application chemical fertilizers(NPK)，and chemical fertilizers plus straw returning(NPKS). Compared with CK，NPK and NPKS treatments significantly reduced soil pH by 1.01 and 0.83 units，respectively，while NPKS treatments increased soil pH by 0.18 units compared to NPK treatments. Compared with CK，the exchangeable acid content of the soil increased by 3.31 and 2.23 cmol/kg in the NPK and NPKS treatments，respectively，and the exchangeable acid content in the NPKS treatment was 1.08 cmol/kg lower than that in the NPK treatment. Compared with CK，the total amount of exchangeable base cations in the soil significantly were decreased by 3.49 and 2.21 cmol/kg in the NPK and NPKS treatments，respectively；the total amount of exchangeable base cations in the NPKS treatment was 1.28 cmol/kg higher than that in the NPK treatment. Compared with CK，the soil organic carbon content were increased by 1.43 and 1.75 g/kg in the NPK and NPKS treatments，respectively；the mineral-associated organic carbon content in the NPKS treatment was 1.56 and 0.89 g/kg higher than that in CK and NPK treatments, respectively. Correlation analysis showed that soil exchangeable aluminum was significantly negatively correlated with exchangeable calcium and magnesium；random forest results indicated that exchangeable calcium and magnesium were the main reasons for the lower content of exchangeable aluminum in soils treated with straw return compared to those treated solely with chemical fertilizers. Long-term application of chemical fertilizers exacerbated the acidification of red soil，while straw return alleviated the acidification of red soil compared to the sole application of chemical fertilizers；the return of calcium and magnesium ions through straw was the main reason for its alleviation of acidification. However，straw return alone couldn’t completely inhibit the acidification of red soil caused by chemical nitrogen fertilizers. Therefore，further research is needed to explore how straw incorporation can be combined with other improvement measures to synergistically amend acidification.