Abstract: In order to find the root causes for vibration of an in-line circulator pump, the unsteady pressure pulsation phenomenon of an in-line circulator pump with a specific speed of 59, which generated by impeller-volute RSI(Rotor-stator interaction), and its influence on pump vibration characteristics, were studied. For this study, a numerical simulation (CFD) tool was applied to get pump performance data, the CFD results were contrasted with experimental results. Once validated, the CFD model was used for simulating the unsteady pressure field existing in the volute of an in-line circulator pump. To understand more, pressure pulsation magnitudes of 43 measuring points inside the volute are obtained by means of numerical calculation, which are a function of flow rate, for several flow rates ranging from 10% to 130% of nominal flow rate. Particularly, at the blade passing frequency, the relationship among the pressure pulsation characteristics of volute, flow rates, and locations of measuring points inside the volute were successfully investigated by CFD. In addition, the vibration test was conducted to capture the vibration amplitudes as a function of flow rate, of pump by using 25 acceleration sensors which distributed inside four detecting areas. The Fast Fourier Transform (FFT) was applied for analyzing the vibration signals. The simulation and experimental results indicated that the amplitude of pressure pulsation reaches the maximum value near the tongue and blade passing frequency 210Hz which is the domain frequency contributed for pressure pulsation. Both amplitudes of pressure pulsation and pump vibration reach the peak value at the blade passing frequency. We concluded that the blade passing frequency is the domain frequency for the pump vibration, and the force of pressure pulsation, which is caused by blade passing frequency, is the main acting force for pump vibration. Simultaneously, under the off-design conditions, the magnitudes of pressure pulsation and pump vibration are higher than the design point. The sequence is 0.1Qd>1.3Qd>1.0Qd. Furthermore, the sequence of vibration amplitudes of four detecting areas is Pipe Support > Motor > Pump housing > Base. The study can be a reference for the design of low vibration in-line circulator pumps.