Objective:To observe the mechanisms of Kangxianling Decoction on renal fibrosis in rats with adriamycin nephropathy. MethodsSD male rats were randomly divided into 7 groups, 8 rats in each group. The normal group was treated with no surgery, and the sham operation group was treated with 0.9% NaCl solution after right nephrectomy. The other 40 SD rats were treated with right nephrectomy and intravenous injection of adriamycin to establish the adriamycin nephropathy model. After successful modeling, the model rats were randomly divided into the model group, Cozaar group, Kangxianling Decoction groups with low-, medium- and high-dose. The rats were treated with the corresponding drugs for 8 weeks. The serum creatinine, urea nitrogen, 24 h urinary protein excretion, mRNA expressions of α-smooth muscle actin (α-SMA) and fibronectin (FN) in kidney tissue were detected, and the renal pathology was observed after HE staining.  Results:Compared with the model group, the α-SMA expression and the FN level were significantly decreased in Kangxianling Decoction group, with statistically significant differences (P<0.05). Meanwhile, the levels of 24 h urinary protein excretion, serum creatinine and urea nitrogen were decreased after Kangxianling Decoction treatment, the renal function was improved and the development of renal fibrosis was delayed, with statistically significant differences (P<0.05). The anti-fibrosis effects of Kangxianling Decoction was enhanced with the increase of dosage, with statistically significant differences (P<0.05).  Conclusion:Kangxianling Decoction can decrease 24 h urinary protein excretion, improve the renal function and pathological histomorphology in a dose-dependent manner. Its mechanism on improving renal fibrosis may be down-regulating the expression of α-SMA and inhibiting the production of FN.