In the light of the unbalanced vibration in the shafting with three-rotor and four-support such as the steam turbine unit and compressor unit, the vibration characteristics of each rotor with mode shape unbalance is developed. First, the dynamics finite element model of shafting with three-rotor and four-support is built. The first and second bending mode shape unbalanced excitations are applied to each rotor of shafting respectively, and the shaft response analysis of shafting are taken. The relationship between the unbalanced excitation type of each rotor and shafting response is confirmed. Then, the rotor simulation experiment rig for shafting with three-rotor and four-support is designed to investigate the support responses and analyze the resonance points, magnitude-frequency and phase-frequency characteristics due to unbalanced excitation. It shows that the vibration characteristics of shafting with added unbalance inside of rotor is determined collaboratively by mode shape and excitation type. Additionally, the overhung rotor vibration characteristics is obvious. It is beneficial to suppress the unbalance vibration fault of this kind of shafting with three-rotor and four-support.