The advent of epidermal growth factor receptor tyrosine kinase inhibitors （EGFR-TKIs） have brought benefits to patients with EGFR gene mutations， which can not only improve the quality of life of patients， but also prolong their survival. However， most patients develop resistance to EGFR-TKIs after taking them for about one year， leading to disease progression. One of the mechanisms of EGFR-TKIs resistance is the conversion from non-small cell lung cancer （NSCLC） to small cell lung cancer （SCLC）， with an incidence of 2%-14%， which is a relatively rare resistance mechanism. Currently， the mechanism of SCLS transformation remains unclear， and the median survival of patients with NSCLC converted to SCLC is approximately 17.8 months. Studying the resistance mechanism of EGFR-TKIs， the occurrence mechanism and clinical characteristics of transformed SCLC is of great significance for the treatment plan and prognosis of transformed SCLC.