【Objective】This study aimed to analyze the characteristics and influencing factors of soil acid buffering capacity (pHBC) in the Critical Zone of forested red soils derived from different lithologies, to provide a theoretical basis for controlling red soil acidification and maintaining the stability of forest ecosystems in southern China. 【Method】The selected soils within the forested red soil Critical Zone are derived from phyllite, red sandstone, and granite. Soil samples were collected at depths of 4.5-6.0 m and were systematically analyzed for pHBC, pH, mechanical composition, mineral composition, as well as the contents of organic matter, iron and aluminum oxides, exchangeable acidity, and exchangeable base cations. Based on soil acid buffering theory, linear regression and random forest modeling were employed to analyze the vertical variation of soil pHBC and its main influencing factors. 【Result】Soil pHBC showed distinct vertical differentiation: it decreased with increasing depth within a certain range, and increased after reaching an inflection point. Within the pH range of 4.4-5.6, the vertical variation of pHBC was primarily governed by soil pH, clay content, and organic matter content, showing a monotonic increase with decreasing pH and increasing clay and organic matter. Under the overall dominance of pH, the pHBC of phyllite-derived soils showed significant positive linear correlations with the contents of organic matter, clay, kaolinite, and crystalline iron oxides. In the red sandstone-derived soils, pHBC correlated positively with the contents of clay, silt, kaolinite, and amorphous aluminum oxides. For granite-derived soils, pHBC was positively correlated with the contents of organic matter, clay, silt, hydromica, amorphous aluminum oxides, crystalline aluminum oxides, exchangeable K+, and exchangeable Na+. 【Conclusion】Soils within the studied red soil critical zone are highly weathered, featuring high exchangeable acidity and aluminum saturation but low total exchangeable base cation content that varied little with depth. Within the 4.5–6.0 m depth range, base cation exchange contributed minimally to pHBC. The primary buffering mechanisms were proton adsorption onto solid surfaces (organic matter, clay, silt, iron and aluminum oxides, kaolinite) and the dissolution of aluminum oxides. Secondary silicate clay minerals, along with minor amounts of primary minerals such as anorthite, albite, and microcline, also provided a measurable acid buffering effect via mineral dissolution reactions.