Allergic pneumonia, also known as exogenous allergic alveolitis, is characterized by major diagnostic criteria including a clear history of antigen exposure, clinical signs and symptoms, imaging studies, alveolar lavage fluid examination, and pathohistological examination. 1. A clear history of antigen exposure before the onset of the disease, such as a history of working in a textile factory and bird feeding. This is very important for the diagnosis of hypersensitivity pneumonitis. 2. Typical clinical symptoms and signs, such as chills and fever, cough, dyspnea, generalized discomfort with chest tightness. End-inspiratory Velcro sounds can be heard on auscultation at the base of the lungs, and some of them have pestle-shaped fingers. 3. Imaging examination, chest high-resolution CT, can be seen in the center of the fine bronchial nodules, patches of ground-glass shadows or with solid changes, gas trapping the formation of mosaic signs and other characteristic manifestations. 4. Alveolar lavage fluid is dominated by lymphocytes, which can make a clear diagnosis. 5. Histopathologic examination can further support the diagnosis, and usually does not require open lung biopsy. The diagnosis can be made by combining the above patient history, typical symptoms and signs, and relevant laboratory, imaging, and histopathologic examinations. Patients with exogenous allergic alveolitis need to seek medical attention and standardized treatment by a physician to avoid delays.