The bleed air overheat detection system for civil aircraft was used to detect the leakage of bleed air duct and prevent continuous leakage of high temperature bleed air exhaust on the structure and equipment. The alarm set-point is usually set according to maximum ambient temperature around sensor. But the different alarm set-points can also be set according to the detection requirement. During the stage of system design, the alarm set-point was set according to the theoretical calculation of ambient temperature. In this paper a new calculation method is established by testing the ambient temperature and correcting the sensor’s theoretical maximum ambient temperature so as to verify the rationality of the alarm set-point.