Abstract: In order to investigate the distribution of the internal flow field of a Francis turbine operating under high load conditions, and analyze the related problems of the unit's flow instability caused by the cavity vortex zone. Based on the finite volume method (FVM), a numerical calculation model for two-phase flow of a Francis turbine is established in this paper. Combined with the SST k-ω turbulence model and Z-G-B cavitation model, the distribution of cavitation phenomena inside the runner and draft tube of the turbine under various high load conditions is explored, and the flow effects of blade cavitation and cavity cavitation inside the turbine under different cavitation number are revealed. It can be seen from the research results that cavitation occurs in different degrees in both the runner and the draft tube when the unit is running in the high load condition, and the cavitation vortex rope in the straight cone section of the draft tube presents the shape of spindle. In addition, the cavitation phenomenon of the cavity cavitation of the draft tube has a periodic volume fraction change, which may lead to the low-frequency amplitude fluctuation in the external characteristic parameters and pressure fluctuation of the turbine, which indirectly causes the low-frequency power fluctuations in the turbine.