The dam body will vibrate during flood discharge, and the flood discharge vibration can stimulate the dynamic response of the dam to a certain extent. Based on the measured data of flood discharge vibration of An arch dam, this paper analyzed the modal characteristics of An arch dam by using ITD modal analysis method, and compared the modal frequency and corresponding vibration mode characteristics of the dam with the results of finite element simulation modal analysis. The comparison of the modal analysis results of the two methods was very consistent, indicating that the modal characteristics of the dam could be effectively analyzed based on the measured dam flood discharge vibration response data, which provides a reference for the evaluation index of the health status of the dam during operation.