Abstract: Numerical simulation with finite element method was used to analyze the unstationary characteristic of inner 3D onflow field for the main steam isolation valve (MSIV), the pipeline frame modal and the valve chamber acoustic modal. Based on the two methods of timeaverage flow analysis and large eddy simulation (LES), this paper tries to find the reason and principle of onflow eddy brought by the reduced aperture. The frequency range for the possible vibration and noise of valve pipeline was obtained, which was further proved by the experiment. The result indicates that, the main frequency of noise source engendered by the onflow eddy is close to the certain order connatural acoustic frequency of valve chamber, subsequently the acoustic chamber resonance acts as the main reason for the overhigh vibration and noise. From the onsite measurement which was acquired by laying acceleration transducers at different locations on the MSIV pipeline under real working condition, the multiplefrequency matches with the acoustic chamber resonance frequency.