In order to instantaneously distinguish the Ct (coefficient of viscous damping) and Kt (coefficient of stiffness), which are both functions of time in an M.C.K. nonlinear system, a new identification method is proposed in this paper. The graphs of the Ct-Kt are analyzed and the dynamic behavior of M.C.K. systems in a Ct-Kt coordinate plane is discussed. This method calculates two adjacent sampling data, the displacement, velocity, and acceleration (which are obtained from the responses of a pulse response experiment) and then distinguishes Ct and Kt of an instantaneous system. Finally, this method is used to identify the aerostatic bearing dynamic parameters, C and K.