Epidermal growth factor receptor-tyrosine kinase inhibitors (EGFR-TKIs) have demonstrated significant survival benefits for advanced non-small cell lung cancer (NSCLC) patients with sensitive EGFR mutations. However, patients with EGFR-TKI treatment often develop acquired resistance subsequently. Transformation from NSCLC to small cell lung cancer (SCLC) is a rare EGFR-TKI resistance mechanism for patients with sensitive EGFR mutations. Herein, we report a NSCLC patient with EGFR exon 19 deletion treated with EGFR-TKI. During treatment, the pathological type of tumor showed transformation from NSCLC to combined SCLC and then to pure SCLC after acquiring EGFR-TKI resistance. Genomic analysis revealed that the EGFR exon 19 deletion, TP53 Y220H mutation, and retinoblastomal transcriptional corepressor 1 (RB1) F755V mutation existed persistently. Immunohistochemical results showed the loss of EGFR and RB1 expression in SCLC. The patient received multi-line chemotherapy with platinum agents and experienced a briefly effective window, but died of aggressive tumor progression. We profiled the transformation from NSCLC to SCLC of this case and pointed out the importance of repeat biopsy in response to EGFR-TKI resistance. Our results showed a novel RB1 F755V mutation which may be associated with RB1 loss. This report summarized the clinical characteristics, mechanisms, and predictors of SCLC transformation, and discussed the treatment after transformation.