An optimization solution of Rayleigh damping coefficients is proposed to construct the damping matrix of structures with closely-spaced natural frequencies based on the seismic response spectrum theory and the complete quadratic combination (CQC) rule. A constrained optimization method is further developed to enforce the arbitrary order mode damping ratio equal to the precise value by Lagrange multiplier method. A curved girder skew through steel suspension arch bridge is analyzed to investigate the characteristics of the proposed method. Firstly, the solution stability of optimal Rayleigh damping coefficients is discussed, Secondly. the effects of the different combinations of optimal objective functions and constraint conditions on optimal reference frequencies and seismic response are compared, and the selection of constraints mode in constrained condition is also discussed. Numerical results show that the seismic response calculation error is smaller while the Rayleigh damping coefficients is obtained from CQC rather than the square root of sum square (SRSS) combination. The constrained mode should be specified as the first order significant mode corresponding to structural seismic responses.