Abstract: A pot experiment was carried out to evaluate the effects of different pH value on physiological indices and tissue structure of flue-cured tobacco Yunyan 85. The results showed SOD activities, palisade tissue/leaf tissue value, palisade tissue/sponge tissue value and proline contents in pH≥3.0 treatments increased with pH value decrease. Further decrease in pH value resulted in a decrease in these parameters. The change trend was an inverted “V”-type and pH 3.0 was their turning point. POD activities in pH≤3.0 treatments were significantly higher than the control and pH≥3.5 treatments, but sponge tissue thickness in pH≤3.0 treatments were significantly lower than the control and pH≥3.5 treatments. Palisade tissue thickness increased from pH 5.6 to pH 3.5 and further decrease in acid pH resulted in significant decrease in the said parameters. Soluble protein contents pH≤3.0 treatments were significantly higher than the control and pH≥4.0 treatments, but they had no significant difference with pH 3.5 treatment. Soluble sugar and leaf thickness were significantly decreased with pH value decrease. pH 3.0 could be the hazard threshold for flue-cured tobacco. Acid pH condition had no significant difference on the ratio of sponge tissue thickness/leaf thickness among the treatments. NR was very sensitive to acid pH condition, and NR activities in pH≤4.5 treatment were significantly lower than the control.