Abstract: For flexible operation of hydro-generator sets, structural stability of Francis turbine impellers under partial load operation is extremely important. This paper presents a static analysis and a modal analysis of the impeller under three typical partial load conditions, based on the one-way fluid-structure coupling method. The results show that under load conditions of 80%, 60% and 40%, large deformation is located at the impeller lower ring, the middle of the tail edge, and the middle of the head edge of the impeller blades respectively. Prestressing has small influence on the runner structure such as an increase of only about 1 Hz in its natural frequency, while water mass has a great impact including a frequency reduction of 15% to 36% but the reduced natural frequencies agree better with the real values. Under the influence of flow disturbance at the outlet of guide blades, the impeller will resonate under these three partial loads, largely in the modes of radial, bending, bending and torsion, and axial rotation deformation. Of all the modes, bending and torsion deformation has the greatest influence likely causing blade fracture. The results are significant to theoretical stability analysis of the flexible operation of Francis turbines.