Abstract: An aeroengine deteriorates in the course of its service.Although its perfomance may be restored to some extent after overhaul,it goes down gradually in general.Based on the statistical information analysis of the test bed performance decay of 500 engines after 0 to 3 overhauls,a concept of an overall performance index is proposed to reflect the overall performance of the engine.According to this concept,a mathematical model of the engine deterioration is presented which is established by means of the exponential smoothing theory.The prediction of the engine deterioration has been made whcih is quite consistent with the practice.The final analysis shows that it is necessary and also feasible to adopt the method of statistical and synthetic analysis for the overall engine performance decay of both randomness and regularity and,what is more,with many parameters.