Abstract: When the mode superposition method is applied to the seismic responses analysis of large-span spatial structures，several problems always become difficult to handle，such as the inefficient calculation and the incapable consideration of earthquake random property． To solve the problems，the seismic response components of large-span spatial structures，including the quasi-static components and resonant components were studied and defined． Based on the seismic response component research，a modified strategy for the mode superposition method of seismic responses of large-span spatial structures was proposed． Through numerical analysis of two typical shell structures， the characteristics of the quasi-static responses and resonant responses and effects of correlation between two components were studied． The distribution features of each response component of large-span spatial structures were summarized．