Abstract: A method about air flow estimation for fan and compressor characteristic drift monitoring in turbofan engine was presented in this paper.Air flow was estimated by solving four flow balance equations,including balance equation with exit pressure of internal bypass and external bypass,relationship of nozzle flow,relationship of enthalpy increase flow.By calculating and determining the symbol parameter of sensor error,the influences of sensor error on flow estimation can be separated.Two percent of drift either in sensor error or in components characteristic was simulated.It indicates that this method can observe the fan and compressor characteristic drift and separate it from sensor error drift.