A combined method of field data analysis and shaft system simulation is utilized in this paper to investigate the vibration characteristics of a Kaplan type hydro-turbine and a semi-umbrella type generator unit with two guide bearings under electromagnetic forces. Vibration data of variable excitation test was analyzed to summarize the vibration characteristics due to electromagnetic forces in the field test. A computational model combined with finite element method(FEM)model for the shaft system and a simplified analytical method for the unbalanced magnetic pull was then established to simulate and validate the dynamic response of the shaft system under the effect of electromagnetic forces. The results show that, for generator units of such type, a significant shift on the axis center can happen after excitation. Meanwhile, the vibration amplitude is increased significantly at the upper guide bearing but slightly reduced at the turbine guide bearing. The simulated results agree well with the field test results, indicating that the simulation analysis is reasonable and the reason of the vibration phenomenon can be well revealed.