Abstract: Partition of operation zones is of great significance to the safety and stability of hydropower units. The conventional partition methods are generally based on stability tests under certain water heads and load conditions, but limited test data lead to a difficulty in the partition to cover all the practical operation cases and thus they are relatively rough methods. This paper presents a fine partition method based on online monitoring data and analysis of stability tests. In a case study of a Francis turbine with severe pressure fluctuations in its draft tube, we first draw head-load pseudo color maps of the unit using fluctuation data and obtain the zones of excessive pressure fluctuations for those measuring points exceeding the standard, and then partition the operation zones according to vortex intensity. Comparison of the areas of excessive fluctuations and the partition results with those of the conventional methods shows that our new fine partition method is superior in identifying the boundaries and ranges of draft tube vortex zones and achieving the partition of total water heads.