The diagnosis of the cause of fever of unknown origin (FUO) is one of the main puzzles for clinicians, especially internists, and it is always a worldwide clinical problem for internists in different countries, and it is the most headache and problematic thing for internists. FUO is a case in which the fever lasts for 2 weeks or more, the temperature exceeds 38.3℃ several times, and the diagnosis cannot be confirmed by at least 1 week of complete history taking, physical examination and routine laboratory tests. Some clinicians have a jingle: “bold” in surgery, “confused” in internal medicine. Here the “bold” refers to the surgeon in many diseases, especially in the abdominal cavity when the diagnosis of certain diseases is not clear, the use of dissection, is a very simple and clear t diagnostic methods. The internal medicine doctor’s “confusion” refers to the feeling of being temporarily helpless when the cause of many difficult diseases such as FUO is not clear, and for these difficult internal diseases, repeated examinations, multiple consultations and case discussions are often required to make a clear diagnosis. It is only after experiencing many “confusions” that clinicians have accumulated clinical experience, which is the process of development and growth of clinicians. For example, although FUO is difficult to diagnose, there are still diagnostic methods to follow. Most data show that in clinical practice, there are three main types of diagnostic tests for febrile diseases of unknown origin: 1. Infectious diseases. Most commonly, cultures of pathogens such as blood, urine, stool, sputum, etc., and bone marrow fluid are often required. Tuberculin test, fertilizer reaction, exophthalmia test, heterophilic agglutination test, condensation set test, pharyngeal test, and blood and fecal smear to check for parasitic eggs are commonly used tests. In addition, imaging examinations such as x-ray and CT are also one of the common measures to diagnose the site of infection. 2.Connective tissue diseases. Frequently available tests are: multiple autoantibodies, multiple rheumatic and rheumatoid factors, serum protein electrophoresis, humoral immune and cellular immune function tests; skin, muscle or electromyography and other related tests. 3.Neoplastic diseases. Imaging examinations such as isotope scan and CT; endoscopy such as bronchoscopy, gastroscopy and enteroscopy; bone marrow, lymph node and corresponding tissue aspiration biopsy or surgical exploration; examination of tumor markers, etc. Most clinical summaries show that FUO often has four major categories of etiology: infectious fever occupies the first place, such as bacterial, viral, and tuberculosis (especially extrapulmonary tuberculosis), the second place is connective tissue disease such as SLE and adult Still disease, and the third place is neoplastic diseases such as malignant lymphoma, etc. These three diseases account for about 80% of FUO. In addition, there are still about 20% of other diseases and those of unknown etiology. Therefore, the diagnosis of FUO needs to be based on detailed medical history information, detailed physical examination information and necessary related clinical clues, followed by purposeful routine examination, which are important diagnostic tools. In addition, the accumulated clinical experience of the physician is also an important factor.