To overcome the overly conservative nature of uniform hazard spectra and the unconservative nature of conditional mean spectra, the composite spectrum that combines the previous two spectra is proposed. The conditional periods are determined based on specific seismic information of the site (including magnitude, epicentral distance, etc.). This allows for the construction of multi-composite spectra that capture the regional seismic characteristics. A composite spectrum, representing the envelope of the corresponding conditional mean spectrum of one earthquake scenario, is ‘moderately’ conservative. By considering the influence of all earthquake scenarios in the region, the multiple mixed spectra are applicable for seismic analysis of all structures within the region. As illustrated by a specific region, the uniform hazard spectra, seismic parameters, and conditional periods of the region are determined following the seismic hazard analysis, and the method for generating multiple composite spectra is presented. Both the composite spectra and the design spectra are used to select actual ground motions. A case study is conducted on a typical cable-stayed bridge and the seismic responses in the longitudinal and transverse directions are compared. It shows that due to the contribution of higher modes, there exist significant differences in the vibration amplitude of different bridge components. The force response (including bending moments and shear forces) of the tower is more sensitive to short-period ground motions. Using design spectra to select ground motions significantly overestimates the longitudinal seismic response of the cable-stayed bridge. The overestimation is 50%, 23%, 38%, and 19% for the beam displacement, tower top displacement, tower base bending moment, and tower base shear force, respectively. It is suggested that the envelope of the mean seismic responses induced by the ground motions selected from each composite spectrum be used as the design seismic response of the cable-stayed bridge. This approach reasonably assesses the seismic demand of the bridge, thereby reducing the cost of the cable-stayed bridge and improving its economy.