ObjectiveTo investigate the mechanism of Baihuan Xiaoyao Decoction （Xiaoyaosan added with Lilii Bulbus and Albiziae Cortex） in alleviating depression-like behaviors of juvenile rats by regulating the polarization of microglia. MethodSixty juvenile SD rats were randomized into normal control， model， fluoxetine， and low-， medium-， and high-dose （5.36， 10.71， 21.42 g·kg^-1， respectively） Baihuan Xiaoyao decoction groups. The rat model of juvenile depression was established by chronic unpredictable mild stress （CUMS）. The sucrose preference test （SPT） was carried out to examine the sucrose preference of rats. Forced swimming test （FST） was carried out to measure the immobility time of rats. The open field test （OFT） was conducted to measure the total distance， the central distance， the number of horizontal crossings， and the frequency of rearing. Morris water maze （MWM） was used to measure the escape latency and the number of crossing the platform. The immunofluorescence assay was employed to detect the expression of inducible nitric oxide synthase （iNOS， the polarization marker of M1 microglia） and CD206 （the polarization marker of M2 microglia）. Real-time polymerase chain reaction was employed to determine the mRNA levels of iNOS， CD206， pro-inflammatory cytokines ［tumor necrosis factor （TNF）-α， interleukin （IL）-1β， and IL-6］ and anti-inflammatory cytokines （IL-4 and IL-10） in the hippocampus. Western blotting was employed to determine the protein levels of iNOS and CD206 in the hippocampus. The levels of IL-4 and IL-6 in the hippocampus were detected by enzyme-linked immunosorbent assay. ResultCompared with the normal control group， the model rats showed a reduction in sucrose preference （P<0.05）， an increase in immobility time （P<0.05）， decreased motor and exploratory behaviors （P<0.05）， and weakened learning and spatial memory （P<0.05）. In addition， the model rats showed up-regulated mRNA and protein levels of iNOS and mRNA levels of IL-1β， IL-6， and TNF-α （P<0.05）. Compared with the model group， Baihuan Xiaoyao decoction increased the sucrose preference value （P<0.05）， shortened the immobility time （P<0.01）， increased the motor and exploratory behaviors （P<0.05）， and improved the learning and spatial memory （P<0.01）. Furthermore， the decoction down-regulated the positive expression and protein level of iNOS， lowered the levels of TNF-α， IL-1β， and IL-6 （P<0.01）， promoted the positive expression of CD206， and elevated the levels of IL-4 and IL-10 （P<0.01） in the hippocampus of the high dose group. Moreover， the high-dose Baihuan Xiaoyao decoction group had higher sucrose preference value （P<0.01）， shorter immobility time （P<0.01）， longer central distance （P<0.01）， stronger learning and spatial memory （P<0.01）， higher positive expression and protein level of iNOS （P<0.01）， lower levels of TNF-α， IL-1β， and IL-6 （P<0.05， P<0.01）， lower positive expression and mRNA level of iNOS （P<0.05）， and higher levels of IL-4 and IL-10 （P<0.05， P<0.01） than the fluoxetine group. ConclusionBaihuan Xiaoyao decoction can improve the depression-like behavior of juvenile rats by inhibiting the M1 polarization and promoting the M2 polarization of microglia in the hippocampus.