The PC-9 cells and PC-9/T790M cells were cultured in vitro. After CRISPR/ Cas9 gene editing technology was used to knock out PD-L1 in the PC-9/T790M cells， the cells were divided into three groups： PC-9 group， PC-9/T790M group and Cas9 PC-9/sgRNA group.The expression levels of PD-L1 protein in the cells in various groups were detected by Western blotting method， and CCK-8 method was used to determine the survival rates of the cells in various groups after 24，48 and 72 h of intervention with 5，10 and 20 mmol·L^－1 gefitinib. In vivo experiments，the tumor volumes in vorious groups were detected after gefitinib intervention， and the pathomorphology of transplanted tumor tissue in various groups was observed.

The Western blotting results showed that the expression level of PD-L1 protein in sgRNA1# group edited by CRISPR/ Cas9 was the lowest compared with PC-9， PC-9/T790M， sgRNA2# and sgRNA3# groups. There were no statistically significant differences in the cell proliferation activities in Cas9 PC-9 /sgRNA group compared with other two groups before drug intervention detected by CCK-8 method （P>0.05）. After 10 mmol·L^－1 gefitinib intervention for 72 h， the survival rates of cells in PC-9 and Cas9 PC-9/sgRNA1# groups were significantly lower than that in PC-9/T790M group （P<0.01）.In vivo experiment， compared with PC-9/T790M group， the tumor volumes in PC-9 group and Cas9 PC-9/sgRNA1# group were significantly reduced after gefeitinib intervention （P<0.05）；moderate to severe cell necrosis was found in PC-9 group and Cas9 PC-9/sgRNA1 group， while no significant changes in transplanted tumor tissue were found in PC-9/T790M group.

Using CRISPR/ Cas9 editing technique to knock out PD-L1 in the drug-resistant PC-9/T790M cells can significantly improve the sensitivity of gefitinib.