Modal series is a new and effective tool which is used in analyzing the nonlinear dynamic systems. With a view to internal structural characteristic of systems, this method could be used to analyze nonlinear modal interaction in order to study the dynamic characteristic of systems. This paper firstly gives some indices and expressions of nonlinear participation factor based on modal series. Secondly, the effect of modal interaction on the dynamic characteristic of systems is opened out by the nonlinear-interaction indices. In the end, the key exciter and its parameters are identified by studying the nonlinear interaction of state variables with the critical inertial modes and combination of modes and the sensitivity of controller’s parameters to nonlinear interaction coefficients.