Abstract: Objective: To explore the mechanism by which the rhubarb-astragalus herb pair(HPRA) inhibits renal interstitial fibrosis(RIF) in rats. Methods: Forty-eight male Sprague-Dawley(SD) rats were randomly divided into six groups with 8 rats per group: a sham group, a model group, a low-dose HPRA group, a mediumdose HPRA group, a high-dose HPRA group and a positive control group(losartan group). Except for the sham group, a rat model of RIF was established in all other groups by unilateral ureteral obstruction(UUO). Blood samples were collected from the abdominal aorta to detect the levels of serum creatinine(Scr) and blood urea nitrogen(BUN). Hematoxylin-eosin(HE) staining was used to observe the renal histopathological changes. Immunohistochemistry(IHC) was performed to detect the protein expressions of α-smooth muscle actin(α-SMA), signal transducer and activator of transcription 3(STAT3), and connective tissue growth factor(CTGF) in the rat renal tissues. Western blotting was applied to detect the protein expressions of vimentin, collagen typeⅠ(Col-Ⅰ) and phosphorylated STAT3(p-STAT3) in the rat renal tissues. Quantitative real-time polymerase chain reaction(RT-qPCR) was used to assess the mRNA expressions of α-SMA, Col-Ⅰ, vimentin, STAT3, and CTGF in the rat renal tissues. Double immunofluorescence staining was conducted to evaluate the co-expressions of CTGF⁺ and STAT3⁺ in the rat renal tissues. Results: Compared with the sham group, the model group exhibited dilated renal tubules and vacuolar degeneration in the rat renal tissues. Meanwhile, the levels of Scr and BUN, and the mRNA and protein expressions of α-SMA, Col-Ⅰ, vimentin, STAT3 and CTGF, the protein expression level of p-STAT3, as well as the co-expression proportion of CTGF⁺ and STAT3⁺ were significantly increased in the model group(P<0.05). All HPRA dose groups showed markedly alleviated renal pathological injury compared with the model group(P<0.05). Meanwhile, the levels of Scr and BUN, the mRNA and protein expressions of α-SMA, vimentin, STAT3 and CTGF, as well as the proportion of CTGF⁺ and STAT3⁺ co-expressions were significantly decreased(P<0.05). Conclusion: HPRA can significantly inhibit the progression of RIF in rats, which may be related to its inhibition of the STAT3/CTGF signaling pathway in renal tissue.