Objective:To observe the effects of Yiqi Yangyin Formula on radiation pneumonitis(RP) and lung function in patients undergoing postoperative radiotherapy for non-small cell lung cancer (NSCLC) (stage IIIa). Methods60 patients undergoing postoperative radiotherapy for NSCLC (stage IIIa) were randomly assigned into treatment group and control group, 30 patients in each group. The control group received routine radiotherapy after operation, and the treatment group received Yiqi Yangyin Formula in addition. Both groups were treated for 28 days. Changes in classification of acute RP (RTOG classification) and in lung function were observed.  Results:①During the trial, there were 1 and 3 drop-outs in the treatment and the control groups respectively; and 56 patients finally completed the trial, including 29 in the treatment group and 27 in the control group.②At 2 months after radiotherapy, there was statistically significant difference in RTOG grade in the treatment group (P,<,0.05), while there was no statistically significant difference in the control group (P,>,0.05) compared to that at the end of radiotherapy. At the end of radiotherapy, there was no statistically significant difference in RTOG grade between the groups (P,>,0.05), and no statistically significant difference in the incidence of RP between the treatment group (72.41%) and the control group (85.19%) (P,>,0.05). At 2 months after radiotherapy, there was statistically significant difference in RTOG grade between the groups; and in the treatment group, the incidence of RP was significantly lower than in the control group (37.93% vs. 85.19%), and the degree of inflammation was significantly lower than in the control group (P,<,0.05). ③There was no significant difference in FVC, FEV,1, or MVV in the groups at the end of radiotherapy compared to that before radiotherapy (P,>,0.05). FVC, FEV,1, and MVV decreased significantly in the control group(P,<,0.05) and they remained stable in the treatment group with no statistically significant differences (P,>,0.05) at 2 months after radiotherapy compared to those before radiotherapy. At the end of radiotherapy, there was no significant difference in FVC, FEV,1, or MVV between the groups (P,>,0.05). At 2 months after radiotherapy, the differences in FVC, FEV,1, and MVV were statistically significant between the groups, significantly higher in the treatment group than in the control group (P,<,0.05). ④Variance analysis of repeated measures data showed that there was statistical significance in the time factor and the time x group factor in intra-group variation of FVC, FEV,1, and MVV (P,<,0.05), and there was no statistical significance in the group factor in inter-group variation (P,>,0.05).  Conclusion:Yiqi Yangyin Formula can stabilize the lung function of patients undergoing postoperative radiotherapy for NSCLC (stage IIIa) to a certain extent, lower the incidence of radiation pneumonitis, and help to reduce the degree of the inflammation.