To investigate the dynamic characteristics of East China Power Grid， which contains UHVAC， UHVDC and multi-infeed HVDC， it is necessary to build the electromagnetic transient simulation platform. For efficiency factors， on the premise of reserving the dynamic characteristics of the original power system， both the unconcerned network and low voltage level grids are reduced. In order to simplify the unconcerned network， the coherency equivalence approach is adopted. The coherency identification is based on the dominant mode of the trajectories based on EEAC theory. The parameters of the equivalent generator are adjusted by weakening the voltage-regulating capability of the exciter and tuning the frequency-regulating capability of the equivalent governor. Differently effective and practical methods are adopted to reduce the low voltage level network by their relevancy to DC's inverter. Simulation results show that the equivalent effect and efficiency are well balanced with the suggested practical dynamic equivalence method.