Pulmonary actinomycosis is a chronic purulent granulomatous disease caused by anaerobic Actinomyces Israelis infecting the lungs. This bacterium is commonly found in the oral cavity, dental caries, and tonsillar crypt in normal people. Most of them develop due to poor oral hygiene and inhalation of secretions containing actinomycete particles. It can also come from hematogenous dissemination or direct spread of abdominal lesions. The organism is cultured on agar under anaerobic conditions and grows spherical colonies. In the tissue, it appears as yellow colored particles, commonly known as “sulfur particles”, which are formed by the entanglement of mycelium. Microscopic examination for gram-positive 0.5 ~ 1.0μm mycelium, surrounded by mycelium in a radial arrangement, the end of the mycelium is expanded rod-shaped. The pathology of the disease is characterized by multiple abscesses, fistulas, granulomatous proliferation and fibrous changes. After invasion of the pathogenic bacterium, it is the first to cause leukocytic infiltration in the tissue, forming multiple small abscesses, which penetrate to form multiple sinus tracts, and sulfur particles can be seen in the pus and sinus tract secretions. Histopathological changes are mainly septic granulomas, with actinomycete granules visible in the abscess, epithelioid cells, macrophages, etc. around the granules, and then fibrous tissue outside. The disease can be classified clinically into facial and cervical, thoracic and abdominal types. The thoracic type may involve the lungs, pleura, mediastinum or chest wall, forming abscesses or coughing up pus sputum with sulfur particles, accompanied by fever, chest pain and chest tightness. The chest radiograph and CT findings are nonspecific and may resemble pneumonia, lung abscess or tumor. Definitive diagnosis relies on fungal examination and the finding of sulfur particles to be meaningful. In this case, there were no typical positive signs, lack of necessary laboratory tests, and misdiagnosis of metastatic carcinoma of the lung by imaging, indicating a lack of knowledge of the disease.