In order to overcome technical flaws of the traditional self-shunt excitation system, this paper introduces a new way of stack trengthen excitation and discusses the electromagnetic transient characteristics of the new type superposition strong excitation circuit, and calculated the excitation transformer capacity. Through the electromagnetic transient simulation, found that superimposed strong excitation can indeed make the excitation voltage and excitation current increase, and the excitation voltage response speed very quickly. Three winding transformer and traditional double winding transformer capacity were computed respectively. The calculation results shows that the superposition of strong excitation winding transformer can reduce three of the excitation transformer capacity.