When the pump-turbine operates at off-design conditions, vortex rope usually occurs in the draft tube and causes low-frequency pressure fluctuations. In this paper, the evolution of the vortex rope and its association with the low-frequency pressure fluctuations under typical operating conditions are investigated. The results indicate that: 1) The frequency of pressure fluctuation is mainly affected by the vortex & rsquo;s rotation frequency in the draft tube without cavitation. While when cavitation occurs, the frequency of pressure fluctuations in the draft tube is primarily affected by the cavitation volume evolution; 2) Under the non-cavitation condition, the overall vortex rope behaves as a spiral vortex rope under part load condition, a slender column under design condition, and pear-shaped vortex rope under full-load condition. The vortex rope directly disappears when cavitation occurs under part-load condition. In contrast, the vortex rope becomes unstable, and changes from a single-phase vortex rope to a multiphase vortex rope under the design condition. While at the full-load condition, the vortex rope becomes slenderer, and the rotation is more noticeable than under the non-cavitation condition. (c) 2021 Elsevier Ltd. All rights reserved.