A clay red soil from Anhui and a sandy red soil from Jiangxi were collected for laboratory incubation experiment to study the effects of applying fulvic acid on ameliorating soil acidity and soil pH buffering capacity. Results showed that applying fulvic acid increased soil pH, and the ameliorating effect increased with the amount of fulvic acid added. Adding 50 g/kg fulvic acid increased soil pH from 4.31 to 5.91 for clay red soil and from 4.97 to 5.97 for sandy red soil, respectively. Moreover, similar to change in soil pH, applying fulvic acid increased soil pH buffering capacity (pHBC), and the more fulvic acid added led to greater increase in soil pHBC. Adding 50 g/kg fulvic acid increased soil pHBC from 20.73 to 35.31 mmol/(kg·pH) for clay red soil and from 7.78 to 23.30 mmol/(kg·pH) for sandy red soil, respectively. In conclusion, applying fulvic acid can not only ameliorate effetively soil acidity of two red soils, but also increase soil pHBC and thus increase soil resistance to acidification, and the effect is better for sandy red soil than for clay red soil.