Objective:To investigate the effects of triptolide on radiosensitization of lung cancer A549 cells and the underlying mechanism.Methods:During June-September 2019, lung cancer A549 cells were treated with different concentrations of triptolide for 24 and 48 hours in Animal Experiment Center, Zhejiang Chinese Medical University, China. The inhibitory effects of triptolide on the proliferation of lung cancer A549 cells were determined using MTT method. Appropriate concentrations of triptolide and double distilled water were added to the experimental and control groups, respectively. The effects of triptolide on radiosensitization of lung cancer A549 cells was determined by colony formation assay. Radiosensitization ratio was calculated. Lung cancer A549 cells were divided into blank control, triptolide, radiotherapy, and radiotherapy + triptolide groups. The effects of triptolide on apoptosis and cell cycle of lung cancer A549 cells were determined by flow cytometry.Results:The 10% inhibitory concentration (IC 10) and half maximal inhibitory concentration (IC 50) of triptolide for treating lung cancer A549 cells were 36.61 nmol/L and 259.38 nmol/L, respectively at 24 hours, and they were 9.05 nmol/L and 61.49 nmol/L, respectively at 48 hours. Triptolide had an radiosensitization effect on lung cancer A549 cells, with the radiosensitization ratio of 1.135. The apoptosis rate in the radiotherapy + triptolide group was significantly higher than that in the radiotherapy [(45.47 ± 8.29)% vs. (5.25 ± 0.59)%, t = 6.847, P = 0.002]. The proportion of lung cancer A549 cells at the G2/M phase in the radiotherapy group was significantly higher than that in the radiotherapy + triptolide group [(27.82 ± 0.96)% vs. (11.98 ± 0.55)%, t = 20.176, P < 0.05]. The proportion of lung cancer A549 cells at the G2/M phase in the black group was significantly higher than that in the triptolide group [(17.31 ± 3.42)% vs. (8.05 ± 0.71)%, t = 3.749, P = 0.02]. Conclusion:Triptolide has a radiosensitization effect on lung cancer A549 cells, and the underlying mechanism may be related to its participation in cell apoptosis and cycle regulation.