In this paper, dynamic behaviors of rotor?slip?bearing system with loosening?rubbing faults are analyzed , and the evaluation method for the support loosening of rotor?bearing systems is proposed. The dynamic model of the local rub?impact for the rotor system with support looseness is established by using nonlinear short?bearing oil film force model, the loose stiffness model and the Hertz contact theory. The dynamic behaviors of the system with looseness and clearance is studied and analyzed. Then, a method of evaluating the bearing looseness for the rotor?sliding bearing under loosening?rubbing faults is proposed based on the nonlinear measurement of dynamic behaviors. The linear approximation kinetic model is obtained by Taylor expansion, and the differences between nonlinear model and its linear approximation model are quantified. Meanwhile, the relationship between looseness gap and nonlinearity degree is established, the influence degree of looseness gap on the system is intuitively reflected, and the looseness state of rotor?sliding bearing system is also evaluated. The results of this paper can give the theory and foundation for the assessment of support loosening in rotor?sliding bearing systems.