With the development of power electronics technology, the proportion of nonlinear load in the power grid is increasing. It is easy for nonlinear load in the power system to produce harmonics which result in harmonic pollution, therefore, it is necessary to make an analysis for its characteristics. In this paper, a method to obtain the nonlinear load characteristic by obtaining the real-time equivalent impedance of the nonlinear load based on the high order differential equation and the signal fitting is proposed. With using improved matrix beam method to fit the port voltage and current signal and putting voltage and current signal into the high-order differential equation which is to constrain port voltage and current, the equivalent reactance parameters of the load can be obtained through mathematical transformation. The characteristics of the nonlinear load are obtained according to the change characteristics of the equivalent parameters of the load. Simulation results show that the method is feasible and the equivalent impedance parameters of the load can be obtained in real time so that the nonlinear load characteristics can be obtained.