The unsteady numerical simulation of francis turbine was conducted with the application of ANSYS software in this study. Three partial opening changing conditions were analyzed under low flow conditions, and the change of pressure fluctuation by the partial load zone to draft tube vortex high part load zone was simulated. With the application of CFD numerical simulation, it is observed that with the change of the guide vane opening, the tail pipe vortex changed from double helical vortex to single helical vortex and columnar vortex, and the variation law of pressure fluctuation frequency and pulsating amplitude between the corresponding working conditions were analyzed, which is very important to study the mechanism of the pressure pulsation of the draft tube.