The finite element method is the major way to analyze the dynamic characteristics of a high speed spindle. However, the behavior of a system obtained by finite element modeling is different from a real system. The errors come from the difference between the finite element model and the real system. Therefore, we propose a modified finite element model method by using modal testing results. The modification can improve the accuracy of an original finite element model and can be used for further numerical analysis. In this paper, a 30,000 rpm speed-booster spindle with build-in motor is used as a study case. The interference fitting effect of a motor rotor is discussed. In this case study, we introduce a feasible modification approach to improve the finite element model. In comparison with results obtained by the modified finite element model and experiments, this improved method shows its practicability.