Abstract: Abstract: Pumped storage hydropower (PSH) can be focused on the transient stability in the field of energy sources in the world. A great challenge can be the hydraulic instability characteristics of PSH units in the anti-S instability zone. The PSH units are prone to enter the anti-S instability zone during low head start, leading to the failure of the grid connection. There was a serious threat to the safe and stable operation of the units. The severe pressure pulsations can be caused by the complex flow evolution in the dynamic characteristics of the unit. In this study, a computational fluid dynamics (CFD) numerical simulation was introduced to explore the start-up process of a pump turbine at low-head in PSH. Experimental verification was also made on the accuracy of the numerical simulation. A dynamic mesh was used to realize the dynamic opening of the guide vanes. A proportional-integral-differentiation (PID) regulation was also introduced. A closed-loop feedback model was established to regulate the opening of the guide vane using rotational speed fluctuations, in order to realize the simulation of the low-head star-up process of a PSH whole flow system. The numerical simulation was focused on the pressure pulsation characteristics in the area of the guide vane and draft tube. The results show that the numerical simulation was in an excellent agreement with the experimental, and the maximum error does not exceed 10%. The PID regulation model was added to simulate the variation pattern of the active guide vane opening. The strong dynamic and static interaction was caused by the speed of the unit. There were the significant mean pressure changes in the vaneless zone, followed by the speed of the unit. The stator-rotor interaction was dominated the variation of the time-averaged pressure dimensionless amplitude and pulsating pressure dimensionless amplitude in the vaneless zone. By contrast, there were the effects of dynamic and static interference on the pulsation amplitude of the pressure in the upstream. The pressure pulsation signal was evenly distributed over the circumference in the area between the stay vane and the guide vane, whereas, there was the uneven distribution along the circumference in the vaneless area. The vortex near the rotor area first appeared in the center of the blade, and then progressed upstream, eventually forming a stable vortex ring structure at the mid-plane position in the vaneless zone. There were the different fluctuation characteristics in the pressure pulsation in the straight cone section of the draft tube in different stages of the start-up process. The pressure fluctuation was more significant in the PID regulation. The comparison of internal flow revealed that the guide vane opening was caused some changes in the distribution and fluctuations of the velocity in the vaneless zone. Significant large-scale vortices were found in the runner inlet and the vaneless zone at the small guide vane opening. The stator-rotor interaction was combined to be responsible for the high amplitude of the time-averaged pressure and pulsation pressure fluctuations in the vaneless zone. The draft tube vortex rope was ever changing from a side strip vortex rope to a spiral vortex rope, and then to a curtain vortex rope during the start-up process. The persistence and dynamics of the vortex rope were induced the uneven pressure radial distribution. The main reasons were attributed to the drastic pressure fluctuations. The findings can provide a strong reference to improve the success rate of the starting pump-turbine at the low head and connecting to the grid.