Abstract: Fever of unknown (FUO) is a common clinical incurable disease. The cause of the disease is complex. Due to lack of specific clinical manifestations and positive results of laboratory tests, diagnosis and treatment are difficult. Although the medical diagnosis and treatment technology is developing rapidly today, the definite diagnosis rate of FUO has not been significantly improved. It is necessary to deepen the understanding of FUO, study its cause and composition, and diagnosis and treatment strategies to improve the diagnosis efficiency. This article reviews the current status and progress of the causes, diagnosis, and treatment strategies of FUO.