Objective To study the projective effect of N-Acetylcysteine on the Expression of Bcl-2/Bax of bleomycin（BLM）-induced pulmonary fibrosis in rats and its mechanism. Methods pulmonaty fibrosis was induced by intratracheal instillation of BLM in rats , the preventing group ( E group, 7 days before molding) using of NAC ,and the treating group (D group, 7 days after molding) using of NAC.5 rats of every group were sacrificed after using of BLM on days 7 , 14 and 28 ,their lungs were harvested for histological studies. Through the transmission electron microscope observation, the immunity histochemistry method observe the situation of fibrosis degree and Bcl-2/Bax expression in every group .Make clear about the curative effect of NAC to pulmonary fibrosis and its possible mechanism. Results Compared with BLM group, the degrees of pulmonary alveolitis and lung fibrosis in E group and D group obviously reduced (P<0.05)，Inhibited of Bcl-2 decreasing and Bax increasing (P < 0.05 )；furthermore ，the effect in E group is more obviously.Conclusion Neither before 7 days nor after 7 days ,applicating NAC can suppress effectively BLM-induced pulmonary fibrosis, through Inhibiting decreased Bcl-2 and increased Bax, NAC effectively inhibit BLM-induced pulmonary fibrosis， the medical effect of giving NAC before 7days is more remarkable.