Abstract: During the gas turbine engine transient,the clearances of the compressor and turbine blade tips differ from those in stable operation because the heat change between gas and components causes the different radial movements of the rotor and case.In order to simulate the real thermodynamics process in the gas turbine engine transient as possible,a model for estimating the variations of the tip clearance and efficiency with time is given.In this model,the typical stage chosen from a multi stage turbomachinery will be decomposed into three components (case,blade and disc),the instantaneous heat growth of each component is calculated individually by means of the different heat transfer model,and the instantaneous centrifugal growths of blade and disc are also estimated,therefore the instantaneous tip clearance are obtained by summing the heat and centrifugal growths of all components.The shifts in efficiency resulted from the change of tip clearance will be predicted by semi empirical formula,with the aid of the tip clearance model of the varations of the tip clearance,efficiency and thrust with transient time have been estimated.The results prove the effectiveness and accuracy of the model.