Curtain wall cable is an important engineering structure. Aiming at the damage detection of curtain wall cable, finite element method was used for the forced vibration analysis of string structures. The dynamic response sensitivity with respect to the physical parameter, such as elemental cross-sectional area, was derived. The sensitivity of the response power spectral density with respect to elemental cross-sectional area under stationary random excitations is obtained using pseudo excitation method. Then finite element model updating method is adopted to identify the curtain wall cable damage. Numerical simulation shows that satisfactory identification results can be obtained from the power spectral density of response data of several accelerometers and the proposed method is insensitive to the measurement noise. It has the potential for practical application.