Lung cancer is currently one of the most common malignant tumors. Some lesions of non-small cell lung cancer (NSCLC) can transform into small cell lung cancer (SCLC) under certain conditions. Clinical reports on such cases are relatively few due to the low transformation rate. This article aims to review the mechanisms, imaging features, treatments, and prognosis of NSCLC transformation into SCLC on the basis of recent research literature, to facilitate the diagnosis of lung cancer.