Abstract: The neurotoxicity related to immune checkpoint inhibitors (ICI) is rare, and the clinical experience remains limited. This paper reported an elderly male patient with advanced small cell lung cancer (SCLC). Following two circles of chemotherapy combined with immunotherapy (serplulimab, 200 mg), the patient developed fatigue, cognitive impairment, motor apraxia, and ataxia. Based on the examination of blood neurotransmitters and brain magnetic resonance imaging, the patient was diagnosed with ICI-associated encephalitis, and his symptoms improved after hormone treatment.