Abstract: 【Objective】This study aimed to explore the effects of different organic nitrogen substitution ratios on soil acidification characteristics of mountain tea plantations in Guizhou， screen appropriate organic nitrogen substitution ratio， providing a theoretical basis for preventing and controlling soil acidification and promoting green and healthy development of tea plantations.【Method】A field experiment was conducted in a typical tea-producing area in Guizhou. Seven fertilization treatments were set up： no fertilization control （CK）， sole application of chemical fertilizer （ON₀）， 20% organic nitrogen substituting chemical fertilizer nitrogen （ON₂₀）， 40% organic nitrogen substituting chemical fertilizer nitrogen （ON₄₀）， 60% organic nitrogen substituting chemical fertilizer nitrogen （ON₆₀）， 80% organic nitrogen substituting chemical fertilizer nitrogen （ON₈₀）， and 100% organic nitrogen substituting chemical fertilizer nitrogen （ON₁₀₀）. The soil samples from the 0-20 cm plough layer were collected before winter fertilization in tea plantations， and the soil pH， exchangeable acid （exchangeable H⁺， exchangeable Al³⁺）， and exchangeable base ions （Ca²⁺， Mg²⁺， K⁺， Na⁺） were measured and analyzed. The relationships between soil pH and exchangeable acid as well as exchangeable base ions were fitted， and the random forest regression was used to analyze the importance of different factors on soil pH variations.【Result】Compared with CK， the soil pH of ON₀ treatment decreased slightly， but the difference was not significant （P>0.05）. Compared with ON₀ treatment， the soil pH in the treatment of organic nitrogen substituting chemical fertilizer nitrogen increased by 0.80%-28.27%， with significant differences observed in ON₆₀-ON₁₀₀ treatments （P<0.05， the same below）. Compared with ON₀ treatment， the contents of soil exchangeable acid， exchangeable H⁺， and exchangeable Al³⁺ in ON₄₀-ON₁₀₀ treatments significantly decreased by 41.13%-68.44%， 55.77%-65.38% and 37.83%-69.13%， respectively. The contents of exchangeable Ca²⁺， exchangeable Mg²⁺， and total amount of base ions in ON₂₀-ON₁₀₀ treatments significantly increased by 20.36%-113.91%， 30.69%-99.01%， and 14.58%-81.84%， respectively. The proportion of exchangeable Al³⁺ accounted for 79.78%-86.50% of the exchangeable acid， and the proportion of exchangeable Ca²⁺ accounted for 63.34%-74.61% of the total exchangeable base ions. Soil pH showed an extremely significant negative correlation （P<0.01， the same below） with contents of exchangeable acid， exchangeable H⁺， and exchangeable Al³⁺， and an extremely significant positive correlation with contents of exchangeable Ca²⁺ and exchangeable Mg²⁺. The results of random forest analysis showed that soil pH was most affected by contents of exchangeable Ca²⁺ and exchangeable Mg²⁺.【Conclusion】Soil pH in tea plantations is mainly affected by the contents of exchangeable Ca²⁺ and exchangeable Mg²⁺. The substitution of organic nitrogen for chemical nitrogen prevents and controls soil acidification by reducing exchangeable acid and supplementing base ions. When the organic nitrogen substitution ratio reaches 40% or above， it can significantly reduce soil exchangeable acid content and increase the total amount of exchangeable base ions， playing a positive role in ameliorating soil acidification in tea plantations.