The precise identification of the damping ratio of a bridge structure is generally challenging. The existing representative damping ratio identification methods in the frequency domain, time domain and time-frequency analysis are summarized in order to study the discrete problem of identifying damping ratios using operational modal analysis (OMA). Reasons for incorrect results in each method are investigated. Then, the identified damping ratios are analyzed by using the OMA of a continuous pre-stressed concrete bridge as an example. Finally, a method to reduce the discreteness of the identified damping ratios is studied. The results show that the damping ratios have higher dispersion than that of the natural frequencies. This problem can be relieved by increasing the sampling time when combining the free vibration response, so the accuracy of the damping ratios would improve. The damping ratios extracted from the ambient vibration responses with lower vibration levels show lower discreteness.