A false signal problem of the bleed air leak detection system in a widely used modem airplane is studied in this paper. It is considered to be a problem of stability of the controller unit. An equivalent circuit is extracted from the study. The circuit is considered to be an alternative bridge. A mathematical model is derived to describe the stability of the circuit. The conclusion is drawn that the sending of the false signal is relevant to not only the sensors, but also the detector. If a parameter in the detector is readjusted, then the problem may be avoided.