Objective:To explore the mechanism of Modified Niduqing Mixture on the renal interstitial fibrosis in rats with unilateral ureteral obstruction(UUO). Methods50 male SD rats were selected. Except 10 rats in the sham-operation group,the UUO model was established in the other rats. After modeling,the rats were randomly divided into the model group,Erbesartan group,Chinese medicine groups with low-dose and high-dose,10 rats in each group. The rats were treated with the corresponding drugs. The urinary levels of N-acetyl β-D-Glucosaminidase(NAG enzyme) and β,2,-microglobulin(β,2,-MG),the serum creatinine(Scr),blood urea nitrogen(BUN),and the expressions of transforming growth factor-β,1,(TGF-β,1,) and α-smooth muscle actin(α-SMA) in renal tissue were detected,and the renal pathology were observed.  Results:①There was no significant difference on the urinary β,2,-MG level among these groups(P,>,0.05). ②Compared with the sham-operation group,the urinary level of NAG on the 14th day and 28th day was increased significantly in the model group,Chinese medicine groups with low- and high-dose,with statistically significant differences(P,<,0.05). The NAG level on the 28th day in the high-dose group was decreased significantly compare with the model group(P,<,0.05). ③The Scr level on the 15th day in the Chinese medicine group with high-dose was decreased significantly compared with the model group and Erbesartan group,with statistically significant differences(P,<,0.05). The Scr level on the 29th day in the Chinese medicine group with high-dose was significantly decreased compared with the model group,with statistically significant difference(P,<,0.05). ④Compared with the Erbesartan group,the BUN level was decreased in the Chinese medicine group with high-dose,with statistically significant difference(P,<,0.05). There was no statistically significant difference on the BUN level among these groups on the 29th day(P,>,0.05). ⑤Compared with the sham operation group,there were statistically significant differences on the expression levels of α-SMA and TGF-β,1, in the model group,Erbesartan group and Chinese medicine groups with low- and high-dose(P,<,0.05). Compared with the model group,there were also statistically significant differences on the expression levels of α-SMA and TGF-β,1, in the Erbesartan group,Chinese medicine groups with low- and high-dose(P,<,0.05).  Conclusion:Modified Niduqing Mixture can reduce the urinary NAG enzyme and inhibit the expressions of α-SMA and TGF-β,1, in the renal tissue,thus alleviate the renal interstitial fibrosis.