Abstract: A method for predicting the off design performance of an axial flow compressor is presented.The fundamental of the calculation is incorporation of three flow phenomena:the annulus flow,the cascade flow and the flow of annulus wall boundary layer.The calculation of the annulus flow is based on the radial equilibrium of mainstream and neglects the existence of cascades and the effects of annulus wall boundary layers,so it aims at calculating the mainstream axial velocity distribution.The calculation of the cascade flow is intended to obtain lift and drag coefficients of cascade airfoil and to calculate the incremental of total enthalpy and entropy across cascades.The calculation of the annulus wall boundary layer provides the blockage effect and the losses caused by annulus wall boundary layer.For obtaining the whole losses of the compressors,the loss models of shoke wave,blade wake and secondary flow/clearance leakage should be coupled with the above mentioned calculations.As examples,the off design performance curves of compressors have been calculated,one of them gives the effect of the annulus wall boundary layer on the compressor characteristics.Additionally,the axial development of the boundary layer momentum thickness and blockage coefficient are also given.The comparison of calculated curves with the rig test results showes that the errors will be allowable in engineering.