Abstract: Objective To explore the influence of autophagy on radiosensitivity of drug resistant lung adenocarcinoma cells. Methods Transmission electron microscopy and fluorescence microscope were applied to assess autophagy activity of parent lung adenocarcinoma cells（SPC-A1） and drug resistant lung adenocarcinoma cells（SPC-A1／DTX）. After the pretreatment of autophagy inhibitor 3-methyladenine（3-MA） with SPC-A1／DTX， Western blotting analysis was used to detect autophagy related protein expression of LC3， p62 and Beclin-1 with or without 6 Gy-radiation. Apoptosis and cell cycle distribution were analyzed by flow cytometry. Cell viability and proliferation were examined by MTT and colony formation assay, respectively. Results The chemoresistant SPC-A1／DTX cells with higher autophagy activity were also radioresistant compared with the parental SPC-A1 cells（P＜0.05）. SPC-A1／DTX cells showed an increase in autophagosome and punctate localization of green fluorescent protein-LC3（characteristic of autophagy） compared to SPC-A1 cells. Irradiation elevated autophagy levels of both cells. Autophagy inhibitor 3-MA combination with irradiation suppressed the expression of autophagy markers, including LC3， p62 and Beclin-1（P＜0.05）. Irradiation-induced G2／M phase delay was increased and S phase arrest was decreased by combination of 3-MA with irradiation（P＜0.05）. The apoptotic rate was elevated when pretreatment with 3-MA before radiation. Conclusion Our results demonstrated that irradiation-induced autophagy provided a cytoprotective mechanism against radiotherapy in SPC-A1／DTX cells. Autophagy contributed to radioresistance of docetaxel-resistant human lung adenocarcinoma cells， and blocking autophagy would be a potential strategy for reversing chemoradiotherapy cross resistance of lung adenocarcinoma patients.