In this study, the influence of valve closing and opening operations to change inner pressure during pump operations on the pulsation boundary condition was investigated to improve the prediction accuracy of pressure pulsation. Vibration experiments were conducted by using an actual size mock-up piping system with 40 m long as found in nuclear power facilities. A drastic change in the boundary condition of the acoustic resonance by a slightly different valve opening ratio to set a different inner pressure was seen in the experimental results. The simulation results by using the method of characteristics showed that the boundary condition of the acoustic resonance changed when the valve opening ratio was changed slightly from 10 % to 15 %. This indicated that the boundary condition of the acoustic resonance had a pulsed change and changed even at the small valve opening ratio, which was approximately 15% though the design threshold value was 25 %. It was concluded that the boundary condition of the acoustic resonance was sensitive to a slight change of the valve opening ratio and this was one of the reasons why it is difficult to predict the pressure pulsation behavior including acoustic resonance accurately in actual plants.