Due to structural stiffness characteristics, there are essential differences in the dynamic behaviors and responses between large-span spatial structures and traditional multi-storey or high-rise buildings. Therefore, it is difficult, sometimes even impossible, to calculate the earthquake responses of spatial structures with the traditional mode superposition of response spectrum method, since the mass participation factors always accumulate very slowly. Aiming at the deficiency of the method, the dynamic characteristics of spatial structures were investigated. The essential characteristics and the deformation mechanism of earthquake responses of spatial structures, including the distribution law of vibration modes, were revealed and concluded. Based on the definition of mass participation factors (MPF), the principle of vibration mode truncation and the mutual theoretical relation of both, a new simplified method for the dynamic computational models of large-span spatial structures was proposed. Then, numerical analysis of a numerical example was conducted to verify this method. The numerical results have indicated that the new simplified method is effective, efficient and accurate for the earthquake response analysis of large-span spatial structures.