【Objective】This study aims to elucidate the effects of sulfuric acid rain acidity on the physiological characteristics of Cunninghamia lanceolata (Chinese fir) seedling roots under nitrogen deposition, providing a theoretical basis for the sustainable management of Chinese fir plantations in acid-nitrogen deposition area.【Method】A pot experiment was conducted using one-year-old Chinese fir seedlings to simulate different levels of nitrogen deposition (no nitrogen addition (N0), low nitrogen addition at 5 g/(m²·a) (N1), and high nitrogen addition at 15 g/(m²·a) (N2))and acid rain (no acid addition (pH 7.0, S0), weak acid addition (pH 4.5, S1), and strong acid addition (pH 2.5, S2)). The experiment investigated the effects of these treatments on seedling root physiology and soil nutrient dynamics.【Result】Under no nitrogen treatment (N0), decreasing pH of sulfuric acid rain significantly reduced root activity and soluble protein content, while increasing root malondialdehyde (MDA) content, peroxidase (POD) and catalase (CAT) activities, and root biomass. Similarly, under no acid rain treatment (S0), nitrogen addition significantly decreased soluble protein content, while root biomass increased with higher nitrogen addition. Low nitrogen addition (N1) had no significant effect on root activity, MDA content, or CAT activity, whereas high nitrogen addition (N2) significantly reduced root activity and increased MDA content and CAT activity. Under N1 and N2 treatments, root activity and soluble protein content were significantly lower in weak acid (S1) than in strong acid (S2) treatments. In the N1 treatment, sulfuric acid rain significantly increased root MDA content, POD, and superoxide dismutase (SOD) activities. Under the N2 treatment, decreasing pH of sulfuric acid rain gradually reduced root MDA content and SOD activity, while POD activity initially decreased and then increased. Additionally, both sulfuric acid rain and nitrogen addition reduced soil pH, with nitrogen addition significantly lowering soil pH under weak acid treatment but showing no significant effect under strong acid treatment. High nitrogen addition alone increased soil nitrate and ammonium nitrogen content (N${\mathrm{O}}_{3}^{-}$-N and N${\mathrm{H}}_{4}^{+}$-N). In the low nitrogen (N1) treatment, weak acid (S1) resulted in the highest soil N${\mathrm{O}}_{3}^{-}$-N content, significantly higher than strong acid (S2). Under high nitrogen (N2) treatment, sulfuric acid rain significantly reduced both nitrate and ammonium nitrogen content. In the no nitrogen (N0) treatment, sulfuric acid rain significantly decreased soil N${\mathrm{O}}_{3}^{-}$-N and available phosphorus content.【Conclusion】Sulfuric acid rain and nitrogen deposition exhibited significant interactive effects on the physiological characteristics of Chinese fir roots. Although the intensity of sulfuric acid rain in China has gradually decreased, the synergistic effect of mild acid rain and nitrogen deposition may still exacerbate soil acidification, reversing the beneficial impact of low nitrogen deposition on root physiology, reducing seedling root activity, and hindering the sustainable management of Chinese fir plantations.