Postoperative fever is categorized into infectious and non-infectious, and fever that occurs after an average of 2.7 days postoperatively is usually infectious. Fever is the most common symptom after surgery. Postoperative fever does not necessarily indicate concomitant infection, but the early or late onset of fever can indicate infection. Non-infectious fever occurs on average 1.4 days after surgery, and infectious fever occurs on average 2.7 days after surgery. So, generally the occurrence of fever after 2.7 days postoperatively is indicative of infection. Risk factors for infectious fever include frailty, advanced age, poor nutritional status, obesity, use of immunosuppressive drugs or pre-existing infectious lesions, etc.; surgical factors include poor hemostasis, residual dead space, tissue trauma, etc.; and also associated with pneumonia, urinary tract infections and other febrile etiologies. Therefore, postoperative fever is relatively complex, time is not the only factor to determine the nature of fever, but also need to be combined with the patient’s own disease, specific surgical operation, postoperative care and other aspects of comprehensive consideration. It can only be said that time is a kind of hint, for example, fever occurring after 2.7 days postoperatively should first be considered to be caused by infection.