This paper proposes the method of antiresonance frequency analysis of multi-input multi-output system. The structural dynamic modification techniques by antiresonance frequency analysis are also applied to reduce the undertest at specimen attachment points on the fixture in environmental vibration test, which is resulted from the inconsistency of antiresonance frequencies at any specified points. Several computer simulations show that the proposed method can remove the undertest problem which is not removed in conventional vibration test control. And the effectiveness of the method is verified with the impact hammer excitation of aluminium fixture model.