Abstract: This study explores the antidepressant mechanism of Radix Bupleuri-Radix Paeoniae Alba herb pair and the core role in Xiaoyaosan. The behavioral indicators were used to observe the effects of herb pair, Xiaoyaosan, and a negative control (Xiaoyaosan remove Radix Bupleuri-Radix Paeoniae Alba) on CUMS rats. The body weight, number of crossing and rearing, sugar preference rate were significantly decreased (P<0.001) and the immobility time of forced swimming test was significantly prolonged (P<0.01) in the model group. Radix Bupleuri-Radix Paeoniae Alba herb pair has significant antidepressant effect, with the number of crossing in low dose group similar to Xiaoyaosan, while the number of rearing and the sucrose preference rate in high dose group are equivalent to Xiaoyaosan, and the immobility time in high and low-doses group are better than Xiaoyaosan. Animal experimentation was approved according to the Committee on the Ethics of Animal Experiments of Shanxi University. Metabonomics results showed 15 biomarkers related to depression, whereas administering Xiaoyaosan or the herb pair can ameliorate 12 or 8 metabolites respectively, compared to the negative control, which can only ameliorate 6 metabolites. The metabolic pathway analysis showed 5 depression-related pathways. The herb pair and Xiaoyaosan exerted antidepressant effects through common 4 metabolic pathways, while the negative control only exerted effects through 3. In summary, behavioral, metabolomics and metabolic pathway analysis revealed that Radix Bupleuri-Radix Paeoniae Alba plays an important role in the biological effect of Xiaoyaosan.