A self-induced instability known as vortex rope enhanced by cavitation usually occurs at part load operation conditions in the draft tube and negatively affects the normal operation of Francis turbines. In the present study, the Francis turbine draft tube was simplified and modeled as a diffuser with swirling flow to analyze and understand the flow patterns of the cavitating turbulent flow with strong vortex oscillation. The numerical simulation used the modified shear stress transport partially averaged Navier–Stokes, i.e., MSST PANS turbulence model, and the Zwart–Gerber–Belamri cavitation model to investigate the flow characteristics of the cavitating vortex rope in the diffuser at different cavitation and swirl numbers. The vortex precession and the cavitation surge predicted by simulations agree with the available experimental data, indicating the applied numerical method is suitable for the present study. The results reveal that a smaller cavitation number and larger swirl number promote vortical flow oscillations, which induce pressure fluctuations in the diffuser. Furthermore, the pressure fluctuations can be enhanced by the increase in swirl number, and the frequency and amplitude of the dominant pressure pulsation component significantly increase with the swirl number. It is also depicted that the interaction between vortex rope precession and cavitation surge directly determines the pressure fluctuations in the diffuser at different operation conditions. The analysis based on the vorticity transport equation indicates that the vortex stretching term has an extremely important effect on vortex production for the cavitating vortex rope evolution under the present operation conditions.