Soil is not only a pivotal link connecting the atmosphere, hydrosphere, biosphere, and lithosphere, but also an important component of carbon cycling. Meanwhile, soil is the largest carbon pool in terrestrial ecosystems, with approximately 1500 Pg of C stored in the upper meter of soil. Even small changes in forest soil composition can result in significant consequences for carbon cycling. Soil organic carbon is easily affected by environmental conditions and its stability plays an important role in forest soil. Although soil microbial biomass carbon (SMBC), one of the soil organic carbon fractions, accounts for only a small fraction of soil total organic carbon (TOC), it is more sensitive to environmental change and its turnover rate is faster than that of TOC. Consequently, SMBC is considered an early indicator of changes to TOC. Many studies of SMBC and TOC responses to acid rain have been carried out in temperate forests in developed countries such as Europe and the United States. However, very few such studies have been carried out in the subtropical forests of southern China, despite the fact that acid rain is a serious environmental threat in this region. We conducted a field experiment to simulate acid rain (SAR) in a monsoon evergreen broadleaved forest (BF) in the Dinghushan Nature Reserve of southern China. We investigated the responses of SMBC, TOC, soil pH, and soil respiration rate to SAR. The SAR treatments included control (CK, pH ≈ 4.5; natural lake water), T1 (pH=4.0), T2 (pH=3.5), and T3 (pH=3.0). The acidic solutions consisted of H₂SO₄ and HNO₃ (1 : 1 mole ratio) in natural lake water. Results showed that pH values of forest topsoil decreased gradually with the increase of acidity, indicating increased soil acidification due to SAR. This phenomenon was consistent with results from other studies conducted at the Dinghushan Nature Reserve. Concentrations of SMBC in June 2011 ((603.76±46.18) mg/kg for CK, (565.41±44.48) mg/kg for T1, (521.58±30.92) mg/kg for T2, (509.49±19.40) mg/kg for T3), December 2011 ((488.92±22.71) mg/kg for CK, (379.65±49.46) mg/kg for T1, (346.08±33.81) mg/kg for T2, (318.00±52.35) mg/kg for T3), and June 2012 ((540.48±39.11) mg/kg for CK, (492.30±43.15) mg/kg for T1, (489.65±51.39) mg/kg for T2, (428.53±49.66) mg/kg for T3) were depressed by SAR, and SMBC contents in T3 treatments were significantly lower than in CK treatments (P < 0.05) at all three sampling times. Meanwhile, changes in soil respiration rates induced by SAR were similar to those in SMBC. Because soil moisture and temperature differed between seasons, all measured variables (soil pH, SMBC, TOC, and soil respiration rates) were higher in the warm-wet season than in the dry season. Due to its relative stability, concentrations of TOC did not significantly differ among the four treatments (P > 0.05). Our results indicate that long-term acid rain is likely to reduce the SMBC content and respiration rate of forest soil; these changes are expected to have positive effects on the accumulation of soil organic carbon. However, the effects of acid rain on TOC storage need to be further investigated.