Pressure oscillations in the axial flow pump impeller are associated with operational stability of the pump unit and fatigue failure of the blade. In order to obtain the characteristics and sources of the pressure oscillations and investigate the effects of adjustable guide vanes (AGV) on pressure oscillations, the CFD method based on the RANS equations and SST k-ω turbulence model was applied to simulate the unsteady flow inside the axial flow pump. The monitor points were located on the impeller blade and the guide vane. The static pressure of each survey point was recorded for 14 rotation periods. The calculated head was performed and supported by experimental data with the maximum relative error less than 4%. It indicates that the geometric model, numerical method, and simulation scheme are reasonable and reliable. Fast Fourier transformation method was used in the pressure signal processing. The main frequency of the pressure oscillations on the impeller blade is the guide vane passing frequency and the main frequency on the guide vane is the blade passing frequency, which indicates that the rotor-stator interaction between the impeller and the guide vanes is the source of the pressure oscillation in studied operating points. After adjustment of the guide vanes, the amplitudes of the pressure oscillations on both the impeller blade and the guide vane are reduced. The flow pattern with different guide vane angle was shown by the streamline, and the vortex flow was partly suppressed. It indicates that the rotor-tator interaction is improved in such operating conditions. The adjustable guide vane provides a valid solution to protect the impeller from potential fatigue failure.