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 Ω under the condition of different values of damping ratio ζ, and then acquire a joint distribution of IEM response with the parameters of ζ and Ω. 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.