Based on the FLAURA and AURA III trials, compared to first- and second-generation epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitors (TKIs), osimertinib provides a longer overall survival benefit for patients with untreated EGFR mutated non-small cell lung cancer. Similar to other EGFR-TKIs, drug resistance is, however, inevitable. The most common mechanism of acquired resistance to first-line osimertinib therapy is the C797S mutation, which accounts for 6% of cases. In view of the current challenges of the development of the next generation of EGFR inhibitors, the mechanism of third-generation targeted drug resistances and targeted strategies are key for further exploration. Our case report discusses a female patient with advanced lung adenocarcinoma carrying the EGFR exon19 E746_A750delinsIP mutation who received osimertinib as first-line therapy and acquired C797S resistance during treatment. The patient was then treated with icotinib for 8 months until the disease progressed. Icotinib may be effective in patients with the EGFR 19del-C797S resistant mutation acquired after osimertinib treatment.