Objective: To investigate the effect of sulfur dioxide (SO₂) on the apoptosis of alveolar macrophage (AM) in lung protection of limb ischemia/reperfusion (I/R) induced acute lung injury (ALI), and to find a new target for the control of inflammatory response.Methods: Twenty pathogen-free, adult male Sprague-Dawley (SD) rats (180-230 g) were used in this study. Five rats were to be used for limb ischemia/reperfusion, then plasma was extracted as ischemia/reperfusion serum stimulation. Fifteen rats were to be used for extracting AM by bronchoalveolar lavage. The AM was isolated and cultured, then the cell count was adjusted to 1×10 ⁶/mL, and randomly divided into the following 4 groups (n=6): control group, I/R group, SO₂ group, and I/R+SO₂ group. The I/R group was given ischemia/reperfusion serum (500 μg/L) to stimulate 6 h; the SO₂ group was given an SO₂ donor, Na₂SO₃/NaHSO₃ [(0.54 mmol/kg) / (0.18 mmol/kg)]; and the I/R+SO₂ group was given the same ischemia/reperfusion serum and Na₂SO₃/NaHSO₃ at the same time. The level of mitochondrial membrane potential, the state of mitochondrial permeability transition pore (mPTP), the rate of AM apoptosis, the expression of Bcl-2 and Caspase-3 proteins were detected by flow cytometry, microplate reader and Western blotting.Results: Compared with the control group, in the I/R group, the ratio of red to green fluorescence and the absorbance decreased significantly, the percentage of apoptotic cells increased obviously, the apoptotic rate was 43.81%±2.40%, Caspase-3 protein expression increased, Bcl-2 protein expression decreased. While compared with the I/R group, in the I/R+SO₂ group, the ratio of red to green fluorescence and the absorbance increased significantly; the apoptotic rate decreased to 37.01%±1.93%, Caspase-3 protein expression decreased, Bcl-2 protein expression increased.Conclusion: Exo-genous SO₂ has the effect of accelerating AM apoptosis by stimulating mPTP to open and mitochondrial membrane potential to decrease; besides, exogenous SO₂ could stimulate AM to secrete more anti-inflammatory cytokines and less inflammatory cytokines. In conclusion, exogenous SO₂ can reduce macrophage apoptosis by inhibiting mitochondrial pathways.