The modal parameters identified from seismic records of concrete arch dams can reveal the variation of actual dynamic characteristics of the structure during earthquake, thus providing important information for seismic response analysis and post-earthquake damage assessment of the structure. In this paper, the basic principles of common modal identification methods were first introduced. Then, the observation data of two earthquakes were used to identify the modal parameters of Longyangxia arch dam by using two different methods of "input-output" and "output-only". Finally, the rationality of the modal identification results in this paper were analyzed by comparing with the engineering experience values and the research results of other scholars. The research results can provide experience and reference for similar projects.