Abstract: The FEM analysis is applied to study the vibration signal distortion due to the excessive motion of the control box fixture in a direction other than the principal vibration direction. The transfer characteristics of the control box fixture are analyzed first. And then the vibration response of a vibration table-fixture-product coupling model based on the measured acceleration data is calculated by the large mass method. The signal transmission distortion is shown to be caused by the fixture. By the thickness optimization, the natural frequency of the fixture is effectively improved. It is shown that the resonant vibration of the fixture in a direction other than the principal vibration direction within the excitation frequency range causes an over-rigorous examination of the product, and the modal contribution analysis can be more targeted to the design or the improvement of the fixture structure.