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When a building is directly struck by lightning, the electromagnetic environment in the structure is very severe, and the conducted and radiated electromagnetic disturbances are dangerous to the information technology equipment in the building. In this paper, instead of the equivalent circuit method and method of moment used by the former researchers, the finite-difference time-domain method is applied to calculate the impulse response in a reinforced steel structure struck by lightning. Besides a reasonable concordance between the calculation results of current distribution among the vertical columns and those of former experiments, the oscillations superimposed on the current flowing along the reinforced bars, and the relation between the oscillation intensity and the soil conductivity are discussed. Furthermore, the role of a ring conductor to stabilize its interior electromagnetic field is discussed in detail.