Abstract: In model tests of Francis turbines, spiral vortex rope phenomena are frequently observed in the draft tube under certain partial load conditions. In addition to the typical vortex rope rotation frequency fv, pressure signals recorded on the draft tube wall often exhibit the rotational frequency fn and its higher-order harmonics (1～5 fn ) as distinct spectral components. Under some operating conditions, these special frequency components show significant amplitudes, yet their underlying generation mechanisms remain subject to various interpretations. This study investigates the internal flow characteristics of a Francis turbine operating under typical vortex rope conditions. By combining high-speed imaging with pressure fluctuation measurements, the effects of unit discharge, unit speed, and cavitation number on the vortex rope structure and pressure pulsation characteristics are systematically analyzed. Special attention is given to the possible origin of the 1～5fn harmonics. The results indicate that unit discharge significantly influences the amplitude of the vortex rope rotation frequency fv: when the spiral vortex rope is clearly visible, the pressure pulsation amplitude at fv increases substantially. In contrast, the effects of cavitation number and unit speed on the amplitude of fv are relatively minor. Additionally, under certain specific conditions, a slender and stable cavitation vortex rope forms in the draft tube, during which the amplitude at fn increases sharply. This phenomenon is believed to be closely related to hydraulic system resonance triggered by the cavitating spiral vortex rope. The special harmonics at 1～5fn are likely attributed to the elliptical cross-section and precession of the vortex rope, whose self-rotation frequency closely matches these observed components.