Paeonia lactiflora and Astragalus membranaceus are two popular traditional Chinese medicines, commonly used in Chinese herb prescription to treat liver disease. The extract prepared from the roots of Paeonia lactiflora and Astragalus membranaceus (PAE) demonstrated better hepatoprotective activity than the herbs used individually as shown in our previous studies. This study was carried out to investigate the effects of PAE on liver fibrosis induced by porcine serum (PS) in rats and to explore its possible mechanisms. Liver fibrosis was induced in male Wistar rats by injection with PS intraperitoneally. The rats were randomly divided into a normal control group, a liver fibrosis model group and a PAE (40, 80, 160 mg•kg-1) treated group. After a 16-week treatment, PAE-treated rats showed significantly reduced liver damage and symptoms of liver fibrosis upon pathological examination. Administration of PAE significantly decreased serum HA, PC III levels, and content of hydroxyproline in the liver tissue of fibrotic rats. It also restored the decrease in SOD and GSH-Px activities and inhibited the formation of lipid peroxidative products during PS treatment. In vitro, PAE also significantly decreased [3H]-thymidine incorporation in hepatic stellate cells (HSCs) stimulated with platelet-derived growth factor-B subunit homodimer (PDGF-BB). Moreover, PAE significantly decreased the expression of PDGF receptor beta (PDGFR-β) and p-ERK1/2, p-p38, p-JNK. The results showed that PAE displays antifibrotic effects in rats induced by PS, the mechanism by which might be associated with its ability to scavenge free radicals, decreasing the expression of PDGFR-β, inhibition of HSC proliferation and MAPK activation. These findings indicate that PAE is a potential agent for the prevention of liver fibrosis.