To scientifically monitor and predict the performance of large-scale tower-beam dampers during operation, the longitudinal vibration of the cable-stayed bridge main beam in natural environment was made equivalent to the problem of the single degree of freedom (SDOF) system under random vibration in this study, and the power spectrum under white noises was deduced. Several acceleration gauges were mounted on the bridge and dampers and field tests were performed three times in three years repetitively. Discrete wavelet transform (DWT) was applied to extract the low frequency vibration data of the main beam. With the measured acceleration power spectrum curve fitted to the formula in SDOF system by the least square method, the damping ratio of the tower-beam damper was obtained approximately. Results showed that after the simplification of the cable-stayed bridge, the system damping ratio of longitudinal vibration of the main beam can be precisely identified. Compared with traditional low-pass filters, the DWT method performs better in conserving the low frequency component. The attenuation of the system damping ratio indicates that the tower-beam damper installed at Sutong Bridge degenerates in a slow process. The field monitor of the tower-beam damper should be carried out regularly, which is beneficial for maintenance guidance and the management of dampers without interrupting traffic, which guarantees the normal operation of long-span cable-stayed bridges.