The invention relates to a method for conducting multi-modal dynamic equivalence on complex electrical power systems except for generators. The method comprises the steps of S1 enabling an external system of a generator set to be studied to be equivalent to a parallel system of an infinite bus and N dynamic units, wherein the infinite bus is expressed as (a formula), and the N dynamic units are expressed as (a formula), S2 in a steady state condition, conducting load flow calculation or on-line measurement to obtain an initial steady-state value of a system, S3 exerting disturbance, recording obtained response quantity, S4 adopting spectral analysis or the prony method to analyze the number N of modals of significant variables at the end of the generator set to be studied, determining the total number N of dynamic equivalence units, S5 according to a multi-modal equivalent model, using data to select an appropriate identification method to identify parameters required to be equaled, and obtaining the dynamic equivalence result. The method better reflects dynamic characteristics of the external system of the generator set to be studied and improves accuracy of optimization design of an excitation controller.