Objective To observe the effects of Yangyin Qingfei Formula on the balance of Th17/Treg in peripheral blood of rats with radiation-induced pneumonia and explore its mechanism of prevention and treatment of this disease. Methods Fifty SD rats of half male and half female were randomly divided into blank group（ C）,model group（ M）,prednisolone group（ J）,TCM group（ Z）,TCM and prednisolone group（ X）. The whole chest of rats in four groups except the blank group was irradiated by a linear accelerator at a dose of16 Gy. From the first day after irradiation,each group was given the intervention. Four weeks after irradiation,10 rats in each group were sacrificed. HE and Masson staining were performed in the middle of right lung,and the whole blood samples were collected for flow cytometry. The pathological changes of lung tissue and the changes of Th17,Treg and Th17/Treg in 4 weeks after radiation were observed in each group. Results There was a significant difference in Th17 ratio between group M and group C at 4 weeks after radiotherapy（ P = 0. 011 < 0. 05）. There was no significant difference in Th17 between any two groups of group M,J,Z and X（ P >0. 05）. Compared with group C,the proportion of Treg in group M was slightly increased,but the difference wasn’t significant（ P =0. 062 > 0. 05）. The proportion of Treg in group J,Z and X was lower than that in group M（ P < 0. 05）. There was no significant difference in Treg between any two groups of group J,Z and X（ P > 0. 05）. The ratio of Th17/Treg in group M and C was increased with statistical difference（ P = 0. 023 < 0. 05）. There was no significant difference in Th17/Treg between any two groups of group M,J,Z and X（ P > 0. 05）. Conclusion Th17/Treg imbalance exists in the peripheral blood of rats with radiation-induced Pneumonia. Yangyin Qingfei Formula can effectively reduce inflammation and fibrosis in the local lung tissues of rats with radiationinduced pneumonia. The mechanism may be related to the inhibition of Treg.