The deterioration of track surface smoothness after seismic action of the track-bridge system is a key factor influencing the post-seismic operating safety of high-speed trains. This study proposes a linear mapping relationship between seismic-induced irregularities and the operation performance of high-speed trains to simplify the calculation of the seismic-induced dynamic irregularity building upon the residual geometric irregularity. It introduces the concept of an equivalent coefficient for seismic-induced dynamic irregularities. Within the study context, the validity of this equivalent coefficient in the post-seismic operation performance analysis of high-speed trains on bridges is confirmed, taking into account the randomness of ground motions. In addition, the impact of ground motion intensity and the structural natural vibration period on the equivalent coefficient for seismic-induced dynamic irregularities is examined. The study findings revealed that the magnitude of seismic-induced residual geometric irregularities in the track-bridge system far exceeds that of dynamic irregularities induced by earthquakes. Damage to the track-bridge system under seismic action primarily presents as residual deformations, with stiffness degradation playing a secondary role. There is a significant correlation between the root mean square velocity of seismic-induced irregularities and the post-seismic operation level in high-speed trains. This correlation is a quantitative basis for establishing the equivalent coefficient of seismic-induced dynamic irregularities. Under identical peak ground acceleration (PGA) conditions, the equivalent coefficient for dynamic irregularities in the track-bridge system during NF (near-fault) earthquakes is considerably lower than that during MFF (mid-far-field) earthquakes. This underscores the notable impact of the velocity pulse effect on the equivalent coefficient of seismic-induced dynamic irregularities. An increase in ground motion intensity and the structural natural period leads to a rise in the equivalent coefficient of dynamic irregularities. Finally, the stiffness degradation effect in critical components of the track-bridge system shows greater sensitivity to the ground motion intensity and the structural natural period.