From the perspective of system stimulation, this paper took estimation of fuel economy in system modification as the subject, to which regard, a correlation model was built based on the analysis of its influencing factors with corresponding simulation tests conducted toward this end, to provide future reference for economical evaluation of system modification. First of all, this paper divided the influence of refitted system into the fluctuation in fuel price and the changes in fuel consumption rate based on the estimation of fuel cost saving of the same. Simulation models for the fluctuation in fuel price and fuel consumption were established according to the stochastic process and aircraft operation & use. Finally, the author put forward an overall framework for fuel economy. Seen from the MATLAB simulation results, the model presented a sound evaluation over the expectation and variance risks of fuel cost saving under future environmental fluctuations to cover the shortage of current studies.