The new cross spectral energy method(CSEM)is proposed for maintaining cable-stayed bridge safe-ty by the measurable output-only vibration response.Damage assessment of real structures is limited by aseries of problems such as unknown ambient excitation forces,errors introduced by system identification,incomplete dynamic measurements,etc.Thus the methodology based on cross spectral energy of eachsubstructure member is derived to meet these challenges.The novel damage index does not require anymodal or parameter identification technology.It can be calculated directly from vibration test data.In or-der to evaluate the efficiency of the presented methodology,a three dimensional(3D)actual cable-stayedbridge model with one or more damaged positions under operational conditions was studied.In order totestify the reliability of damage detection method,the response data was polluted by the random noise.Itis proved that the proposed method can successfully localize all damage cases even in noisy data.Withthe help of examples,the CSEM can potentially be applied as a nondestructive evaluation technique(NDT)for on-line health monitoring of cable-stayed bridges with minimum disruption of its operations. The new cross spectral energy method (CSEM) is proposed for maintaining cable-stayed bridge safe-ty by the measurable output-only vibration response. Damage assessment of real structures is limited by a series of problems such as unknown ambient excitation forces, errors introduced by system identification, incomplete dynamic measurements, etc.Thus the methodology based on cross spectral energy of eachsubstructure member is derived to meet these challenges.The novel damage index does not require anymodal or parameter identification technology. It can be calculated directly from vibration test data. In or-der to evaluate the efficiency of the presented methodology, a three dimensional (3D) actual cable-stayedbridge model with one or moreredredumbers under operational conditions was studied. Order totestify the reliability of damage detection method, the response data was polluted by the random noise. It is proved that the proposed method can successfully localize all damage cases even in noisy data. Help the examples of the CSEM can be applied as a nondestructive evaluation technique (NDT) for on-line health monitoring of cable-stayed bridges with minimum disruption of its operations.