To observe the curative effect of Maxing Shigan Decoction on pneumonia model mice，and to explore its effects on the level of costimulatory molecule CD137. Method:A total of 60 SPF-free young mice were randomly divided into blank group，model group and Traditional Chinese Medicine group with 20 mice in each group. The mice in the model group and Traditional Chinese Medicine group were given Klebsiella pneumoniae suspension by dripping into the nasal cavity，the mice in the blank group were given the same dose of normal saline nasal drops，the mice in Traditional Chinese Medicine group were given Maxing Shigan Decoction intragastric administration for 24 hours，one dose per day，for 7 days. The general activity，peripheral blood (WBC)，polymorphonuclear hormone (PMN)，interleukin-6 (IL-6)，interleukin-8 (IL-8)，lung histopathology and the concentration of costimulatory molecule CD137 in peripheral blood mononuclear cells were compared. Results:There were no positive signs in the blank group mice，but the model group mice and Traditional Chinese Medicine group mice showed mental dispirity，weight loss，decreased activity，and decreased fur gloss，and Traditional Chinese Medicine group mice were better than the model group mice. WBC，PMN，IL-6 and IL-8 in the peripheral blood of the model group mice and Traditional Chinese Medicine group mice were higher than the blank group mice，the difference was statistically significant (P<0.05)，Traditional Chinese Medicine group was better than the control group (P<0.05). There were a lot of inflammatory cells infiltrating around bronchi，interlobular and alveolar septum，and some fibrosis in the model group and Traditional Chinese Medicine group. Traditional Chinese Medicine group improved significantly compared with the model group. The expression of costimulatory molecule CD137 in peripheral blood of mice in model group and Traditional Chinese Medicine group was higher than that in blank group (P<0.05). The level of costimulatory molecule CD137 in Traditional Chinese Medicine group was lower than that in model group (P<0.05). Conclusion:Maxing Shigan Decoction has obvious clinical effect on pneumonia，and its mechanism may be related to the inhibition of the secretion of costimulatory molecule CD137.