Objective To investigate the possible mechanism of“treating liver by nourishing spleen”in the treatment of liver fibrosis with reference to the effect of Xiaoyao powder and its separated recipe on gut microbiota and the level of portal endotoxins. Methods A total of70 healthy Wistar rats were randomly divided into blank group( 10 rats),model group( 20 rats),experimental group( 20 rats),and control group( 20 rats),and tail vein injection of bovine serum albumin was performed for 8 weeks to establish a rat model of immune liver fibrosis.The rats in the experimental group were given Xiaoyao granules by gavage,and those in the control group were given Xiaoyao granules without the spleen- strengthening traditional Chinese medicines Atractylodes macrocephala Koidz.,Poria cocos,ginger,and Radix Glycyrrhizae Preparata by gavage. Serum aminotransferases,liver pathology,portal endotoxins,and the enterobacterial repetitive intergenic consensus( ERIC)- PCR fingerprint of gut microbiota were observed in each group. The analysis of variance was applied for comparison of continuous data with homogeneity of variance between multiple groups,and the least significant difference t- test was used for further comparison between any two groups; the Tamhane's method was applied for data with heterogeneity of variance; the Pearson correlation analysis was used for correlation analysis. Results Compared with the blank group,the model group showed changes in the diversity and structure of gut microbiota and an increase in the level of portal endotoxins( 0. 421 ± 0. 170 EU / ml vs 0. 784 ± 0. 180 EU / ml),which showed significant differences between these two groups( P < 0. 01),and the level of portal endotoxins was positively correlated with collagen area percentage in liver tissue( r = 0. 736,P < 0. 01). Compared with the model group,the experimental group had significantly reduced levels of alanine aminotransferase( ALT),aspartate aminotransferase( AST),and portal endotoxins( 73. 25 ± 10. 90 U / L vs 59. 84 ± 9. 60 U / L,135. 36 ±31. 41 U / L vs 107. 43 ± 17. 71 U / L,0. 784 ± 0. 180 EU / ml vs 0. 576 ± 0. 220 EU / ml,P < 0. 01 or P < 0. 05),and the rats in the experimental group had similar diversity and structure of gut microbiota as normal rats. The levels of ALT,AST,and portal endotoxins showed no significant differences between the experimental group and the control group( P > 0. 05). Conclusion The rats with liver fibrosis experience intestinal dysbacteriosis which causes the increase in portal endotoxins,and it is one of the mechanisms for the pathogenesis of “liver disease affecting spleen”during the pathological process of chronic liver injury. Xiaoyao powder can relieve liver fibrosis,partially restore the normal structure of gut microbiota,and reduce endotoxins,while the removal of the spleen- strengthening drugs will weaken the protective effect of Xiaoyao powder on the liver. This suggests that restoration of gut microbiota and reduction of portal endotoxins may be an important mechanism of “treating liver by nourishing spleen”in the treatment of liver fibrosis.