Abstract: In order to formulate a simple and exact expression for studying rotor bearing sealing system,the structural dynamic characteristics of brush seals were analyzed.On the basis of the elastic theory,this paper establishes the nonlinear dynamic equations describing the actions between the ring seal and the rotor,in which nonlinear friction between the ring seal and the rotor was considered.Then an approximate expression of the nonlinear supporting stiffness was derived,which provides the mathematical model and method for the study of nonlinear dynamics of rotor bearing sealing system.Finally the correctness of the expression was proved by using numerical simulation.