Abstract: A general model for predicting the thermodynamic performance of variable geometry axial compressors has been developed,based on stage-stacking model.The model is combined with the method,with which off-design operating points of gas turbine engines can be predicted.The simulation program is established for predicting variations in engine surge margins.An example on the use of the model and method is given to prove their validity.The optimization is obtained to predict the effects of combined variable geometry of fan inlet guide vane,mixer and nozzle area on the performances and surge margins of the engine.