Although each rotor is balanced well, unbalance vibration fault still exists for the shafting with three supports (i.e., steam turbine, compressor unit, etc.). The effect of residual unbalance phase difference on vibration characteristics of shafting with three supports is investigated in this paper. A finite element model is developed for shafting with two spans and three supports to analyze the steady-state response. Additionally, the combination of residual unbalances phase difference among multi rotors is presented. The relationship between the supports vibration and the residual unbalance phase difference among multi rotors are confirmed. The phase difference of shafting with residual unbalance mostly affects the shafting vibration in the high speed, and the minimum vibration amplitude for the shafting is from 0-180. Obviously, it is different from the traditional shafting with four supports. Finally, the vibration experiment for the phase difference of shafting residual unbalances is performed to validate the simulation predictions. It is a new way to suppress the unbalance vibration fault for this kind of shafting with multi rotors.