The display of available fuel quantity in the cockpit is one of the important functions of the civil aircraft fuel system. Pilots should be aware of the available fuel quantity throughout the entire flight process. Under normal circumstances, the available fuel in the fuel tank should be sufficient to support the aircraft's arrival at the destination. If the available fuel may not be sufficient to support the aircraft's arrival at the destination, it will be considered unsafe.This article analyzes the requirements for setting fuel level low alarms in CCAR25, FAR25, and CS25, as well as the requirement for airlines/pilots to declare “minimum fuel” in CCAR121. It introduces the necessity of setting fuel level low alarms in civil aircraft and the principle of calculating alarm thresholds. Based on the operational scenarios of civil aircraft, carry out two calculation methods and factor analysis for fuel level low warning thresholds in “waiting” scenarios. Taking a three tank civilian aircraft as an example, calculate the low fuel alarm threshold for two scenarios, and after analysis, obtain the final low fuel alarm threshold. Propose pilot operation suggestions after the aircraft triggers the fuel level low alarm, which can provide reference for the requirement analysis of fuel level low alarm in civil aircraft fuel systems.