Abstract: A method was developed to simulate turbofan component characteristics.Nonlinear simplex optimization was used while keeping the deviation of engine performance and working process parameters a minimum as objective function and component coupling coefficients as optimized parameters.A fault diagnosis equation for the twin-spool mixed flow turbofan engine was established based on the simulation and gas path analysis.Investigation shows that the symptom parameters selection and their relatively deviations exert great influence on the diagnostic accuracy.The symptom parameters and their relative deviations should be kept not greater than one third of their threshold values.Relative errors between the deviation of the real fault symptom parameters and of the fault derived from the model are controlled less than 5%.Inaccurate fault isolation does not appear while the developed model was applied to the diagnosis of the available fault cases.