Abstract: For correct and wide application of reduced thrust takeoff technique, the temperature characteristics ofhigh bypass ratio turbofan engines and the takeoffperformance of large aircraftswere analyzed. Assumed temperature is the equivalent ambient air temperature determined by the actual takeoffmass ofan aircraf.t The necessary conditions ofreduced thrust takeoffare that the actual takeoff mass is less than themaximum takeoffmass of an aircraft and that the assumed temperature is higher than the reference temperature. The rotation speed of the fan under assumed temperature reduced thrust takeoff is calculated based on the analogical condition of engine.