ObjectiveTo investigate the antitumor effect and mechanism of Guiqi Yiyuan ointment on Lewis lung cancer mice based on the extracellular regulatory protein kinase （ERK） signaling pathway. MethodsA Lewis lung cancer mouse model was established. Except for the blank group， the model mice were randomly divided into the model group， Guiqi Yiyuan ointment low， medium， and high dose groups， and the extracellular ERK1/2 inhibitor group， with 10 mice per group. The Guiqi Yiyuan ointment was administered by gavage at doses of 1.75， 3.5， 7.0 g·kg^-1·d^-1 for the low， medium， and high dose groups， respectively. The ERK1/2 inhibitor group was given the ERK1/2 inhibitor LY3214996 （100 mg·kg^-1·d^-1） by gavage. The treatment was administered for 14 consecutive days， after which samples were collected. Tumor histopathological changes were observed using hematoxylin-eosin （HE） staining. Transmission electron microscopy was used to observe ultrastructural changes in tumor cells. Immunofluorescence was performed to measure the phosphorylation of ERK1/2 （p-ERK1/2） and the expression of programmed cell death ligand-1 （PD-L1） in tumor tissues. Western blot and real-time quantitative polymerase chain reaction （Real-time PCR） were used to detect the expression of p-ERK1/2， PD-L1， the autophagy marker Beclin-1， the autophagic protein p62， and the microtubule-associated protein light chains LC3Ⅰ and LC3Ⅱ at both the protein and gene levels. ResultsCompared with the model group， the average tumor weight was significantly reduced in the low and medium dose groups of Guiqi Yiyuan ointment （P<0.05）， and markedly reduced in the high dose and inhibitor groups （P<0.01）. Tumor cells in all treatment groups became progressively irregular， with ruptured nuclei and expanded areas of cell disintegration and necrosis. The number of organellar ablations in tumor tissues increased， and the number of autophagic vesicles also increased in all groups. The mean fluorescence intensity of p-ERK1/2 and PD-L1 was reduced in the low and medium dose groups of Guiqi Yiyuan ointment （P<0.05）， and significantly reduced in the high dose and inhibitor groups （P<0.01）. The mRNA expression of ERK1/2， PD-L1， Beclin-1， and p62 was reduced in the medium dose group （P<0.05）， while LC3Ⅰ/Ⅱ mRNA expression was elevated （P<0.05）. In the high dose and inhibitor groups， mRNA expression of ERK1/2， PD-L1， Beclin-1， and p62 was significantly reduced （P<0.01）， while LC3Ⅰ/Ⅱ mRNA expression was significantly increased （P<0.01）. Protein expression of p-ERK1/2， PD-L1， Beclin-1， and p62 was reduced in the medium dose group （P<0.05）， and LC3Ⅰ/Ⅱ protein expression was elevated （P<0.05）. In the high dose and inhibitor groups， protein expression of p-ERK1/2， PD-L1， Beclin-1， and p62 was significantly reduced （P<0.01）， while LC3Ⅰ/Ⅱ protein expression was significantly elevated （P<0.01）. ConclusionGuiqi Yiyuan ointment may inhibit the activation of the ERK signaling pathway， downregulate the expression of p-ERK1/2， promote autophagic flux in tumor cells， and regulate the expression of PD-L1， thereby exerting an inhibitory effect on tumor growth in Lewis lung cancer mice.