The characteristics of unbalance of turbine generator rotor system are studied by use of modal analysis and Riccati transfer matrix method. The study shows that the superimposition of several lower mode shapes is approximately equal to the superimposition of all the mode shapes. The result shows, when the unbalance is located on a segment of rotor, the mode shapes of this segment and nearby segments are easily excited. The unbalance characteristics of a 200?MW turbine generator rotor system are calculated in this paper.