There is no set time for how long a pediatric fever of 40 degrees lasts dangerously, it varies from person to person. In itself, a fever of 40 degrees has a certain degree of danger, fast may be about 10 minutes will lead to febrile convulsions, convulsions and other dangerous situations. Pediatric fever is to seek medical attention in time, especially fever of 40 degrees, belongs to the category of high fever, need to lower the temperature treatment in time, do not wait for the danger before seeking medical treatment. If the treatment has not been carried out to deal with, it may lead to pediatric febrile convulsions, convulsions, damage to brain cells and so on. Children with a fever of 40 degrees, need to consult a doctor in time, follow the doctor’s instructions to take ibuprofen and other drugs to reduce the fever, at the same time with the physical cooling, such as the head of the antipyretic paste, warm water baths and so on. At the same time, we should consult the doctor in time to clarify the cause of the disease, and treat the cause of the disease. If acute upper respiratory tract infection combined with bacterial infection, need to take antibiotics such as amoxicillin and other antibiotic treatment.