Abstract: While the synthetic weighted method is used to calculate the synthetic exponent characterizing the overall engine performance,the weights of every parameter are difficult to determine.To solve this problem,a method of chaos variable was introduced.The synthetic exponent optimized by chaos variable is more sensitive and accurate than the one gained by the expert investigation method in reflecting the overall engine performance change. This method improves the successful rate of identifying whether the engine performance is normal or not,and can be used to detect the potential incipient fault,thus prevents the spread of the fault.The validity of this method is verified by monitoring a turbofan engine.