Objective To investigate the effects ofYangyin Yiqi Mixture on pulmonary fibrosis caused by bleomycin in rats;To discuss its possible mechanism.Methods SD rats were randomly divided into normal control group, model group, dexamethasone acetate group, low-dose, middle-dose and high-doseYangyin Yiqi Mixture groups. Bleomycin was used to establish pulmonary fibrosis rat models through endotracheal instillation. One day after molding, each medication group received gavage with relevant medicine. Lung tissues were taken on the 7th, 14th and 28th day. Masson staining was used to observe pathological change;alkalinehydrolysis was used to detect HYP content;Western blot was used to detect the protein expressions of MMP-2 and TIMP-1.Results Compared with the normal control group, on 7th day, collagen in lung tissue of rats in model group markedly increased;the content of HYP in lung tissue significantly increased (P<0.01). On 28th day, collagen was deposited diffusely and the alveolar was destroyed;content of HYP in lung tissue increased significantly (P<0.01). Compared with model group, deposited collagen in lung tissue was alleviated, and the content of HYP were remarkable reduced in middle and high doseYangyin Yiqi Mixture groups and displayed dose-dependent. Western blot results showed that the protein expressions of MMP-2 and TIMP-1 increased quickly compared with the low expression of normal lung tissue in the 7th day of molding (P<0.05), and reached high expression in the 28th day. Compared with the model group, the level of MMP-9 in high-dose Yangyin Yiqi Mixture group decreased significantly in the 14th day;the expression of TIMP-1 decreased significantly in the 7th, 14th and 28th day (P<0.05).ConclusionYangyin Yiqi Mixture can effectively resist pulmonary fibrosis, which may be realized by inhibiting the expressions of TIMP-1 and MMP-9, regulating and promoting collagenic hydrolysis and restraining deposition of collagen.