Small cell lung cancer (SCLC) accounts for approximately 15% of all lung cancers and is on the rise annually. It is characterized by low differentiation, high malignancy, and rapid growth. Consequently, treatment options are limited, and the patient's prognosis is poor. The emergence of immunotherapy has partially improved the survival and prognosis of SCLC patients. However, a unique response known as "pseudoprogression" during immunotherapy has raised concerns. The occurrence of tumor enlargement despite a positive response to immune checkpoint inhibitor therapy undoubtedly affects the assessment of clinical drug efficacy and the selection of subsequent treatment strategies. In this article, we analyze a clinical case of pseudoprogression in a patient with SCLC who received immune therapy (Durvalumab). Currently, there is insufficient evidence-based medicine to guide the diagnosis, differentiation and subsequent treatment strategies for pseudoprogression in SCLC following immunotherapy. Through this case report and literature review, we aim to provide diagnostic and therapeutic insights for the clinical use of immunotherapy in advanced SCLC. Additionally, we hope that fellow readers of this article can engage in further collaborative discussions through more clinical research.