In order to evaluate the image motion reliably, a numerical analysis method was promoted for calculations of jitter transfer functions between input disturbances and output image motion. Firstly, Kistler table is applied to measure the disturbances of flywheel and precise test data of each disturbances are achieved. Secondly, a flywheel is assembled in a high-fidelity satellite prototype and jitter-affected image motion data are obtained from the prototype. Thirdly, a series of Linear System of Equations (LSE) whose variables are the transfer functions are constructed and solved with the help of harmonic data of flywheel disturbances and image motion. At last, by applying the calculated transfer functions, the evaluation of image motion of another flywheel which comes from the same product series is completed. With the reference to the test data, the evaluated image motion performs the same harmonical characteristics, the relative errors of typical responses in evaluated image motion are less than 10% and most absolute errors of rest parts are no more than 0.1 pixel. Compared with theoretic modeling method, the numerical method is faster and more reliable.