OBJECTIVE To determine the effect of Kai-Xin-San(KXS)combined with fluoxetine on the intestinal flora and the expression of inflammatory factors in chronic unpredictable mild stress(CUMS)depression mice and to elucidate the antidepressant mechanism of regulating"intestine-brain"axis.METHODS CUMS depression mice model was established and the effect of com-bined medication on improving depression-like behaviors of mice was evaluated by determination of sucrose preference rate,immobile time of tail suspension and forced swimming.Additionally,the levels of inflammatory factors IL-6,IL-1β,TNF-α and LPS were de-termined in cortex,serum and intestine using the ELISA method.The composition of intestinal flora in mouse feces was analyzed by 16S rRNA sequence sequencing.Furthermore,Western blot assay was utilized to determine the expression levels of intestinal barrier proteins such as ZO-1,Occludin and Claudin-5.RESULTS The combination of KXS and fluoxetine resulted in a significant in-crease in sucrose preference rate(P<0.01)and decreased immobile time of tail suspension and forced swimming(P<0.05,P<0.01)in CUMS mice.The antidepressant effect of KXS combined with middle dose of fluoxetine was equivalent to that of high dose of fluoxe-tine alone.Meanwhile,the combination could significantly inhibit the up-regulation of inflammatory factors in the cortex,serum and small intestine of model animals(P<0.05,P<0.01).Intestinal flora analysis showed that the combination could improve the ratio of Gram-positive bacteria to negative bacteria in the intestinal tract of model animals,and the improvement of the relative abundance of intestinal bacteria Lachnospiraceae,Bifidobacterium,Ruminococcus,Blautia,Eubacterium,Intestinimonas,Erysipelotrichaceae,Alis-tipes,Desulfovibrionia and Coriobacteriaceae UCG-002 and so on in the model animals was significantly related to the alleviation of de-pression-like behavior and the down-regulation of cortical inflammatory factors(P<0.05,P<0.01).Furthermore,the combination treatment could significantly up-regulate the expression of intestinal barrier protein(P<0.01).CONCLUSION KXS combined with fluoxetine can alleviate depression-like behavior and reduce fluoxetine dosage in CUMS model animals.The combination of the two drugs may regulate the composition of intestinal flora,inhibit the expression of intestinal inflammatory factors,up-regulate intestinal barrier proteins,and thus reduce the expression of serum and central inflammatory factors,which may be the mechanism of their regu-lation of the gut brain axis in antidepressant action.