An impact energy model (IEM) is proposed for the research of nonlinear rub-impact assessment in this paper. According to the characteristic of the model, IEM index is given to evaluate the severity or probability of rub-impact fault. We investigate the trend of variation of IEM index response to the rotation speed ratio under the condition of different values of damping ratio , and then acquire a joint distribution of IEM response with the parameters of and . Combining the IEM model, a concept of sensitive area is proposed. The sensitive area provides an excellent reference for operating rotor system and a guidance and assessment for designing rotor system.