In this paper, for the high-power dynamic loads, such as traction substations, the quasi-steady state and dynamic state mathematical models of the dynamic load signal random process are first established. Then, based on the second empirical mode decomposition method, the dynamic load current and power random signals of the collected traction substation are decomposed on a small time scale. And the quasi-steady state of slow fluctuation and the dynamic state of random fast fluctuation are obtained. The decomposition results prove the correctness of the model structure and the effectiveness of the decomposition method. The method provides a basis for determining the dynamic error characteristic test signal of the energy meter.