The damping makeup of large span cable-stayed bridges is complicated. The damping matrices of such structures cannot be easily accurately determined. On assumption of the damping coefficients of substructures in cable-stayed bridge (such as girder, tower, cable and support et al) are constants, a method to estimate the damping coefficients and damping matrices of such structures is presented. The method is based on several known modal damping ratios and the calculation of complex eigenvalue and eigenvector utilizing the state-space methodology. Numerical simulation was carried out by the example of Tatara Bridge. Through comparison between the calculated damping ratios utilizing the approximated damping matrices and the field test data, the method is proved valid.