Abstract: Thoracic SMARCA4-deficient undifferentiated tumor (SMARCA4-UT) is a rare invasive tumor that is more common in middle-aged and elderly male smokers, the main feature of the tumor is the loss of expression of the SMARCA4 gene. Most patients are already in the late stage of the disease when they seek treatment, and chemotherapy is often ineffective, the disease progresses rapidly, and the median survival time is about six months. ¹⁸F-fluorodeoxyglucose(FDG) PET/CT imaging plays an important role in reflecting the imaging characteristics, staging, and prognosis of lesions. The authors reported a case of the thoracic SMARCA4-UT with gastric metastasis, and analyzed the characteristics of the disease from clinical symptoms, ¹⁸F-FDG PET/CT imaging, histopathological examination, and prognosis, in order to provide relevant information for clinical reference.