For seismic control of arch bridges, a model reduction of longspan arch bridge was implemented based on modal analysis. As for the critical mode selection, an approach based on the maximum modal displacement was presented. This approach takes into consideration the effect of external seismic excitation and is more reasonable than that only considering dynamic bridge characteristics based on a modal contribution ratio. The time domain and frequency domain analysis methods were used to verify the simplified model of the Nimu arch bridge in Tibet as an example. The numerical results show that the method of maximal modal displacement better analyzes longspan arch bridges when multisupport seismic excitation must be considered. The reducedorder system also is more in line with the performance of the original model.