Abstract: Under the combined action of various loads and material aging, the structure of concrete dam is prone to damage, even damage. Based on the vibration measurement under environmental excitation, this paper analyzes the dynamic characteristics of the structure, evaluates the health status of the structure, and predicts its durability and safety, which is especially suitable for the overall assessment of the health status of large structures such as concrete dams. In this study, the acceleration sensor was used to carry out the vibration test of an arch dam under environmental excitation, and the IPAP-HJAD method for automatic modal parameter identification was formed by combining the improved preference affinity propagation (IPAP) algorithm with the Hankel matrix joint approximate diagonalization (HJAD) method. This method eliminates the influence of outliers on the selection of modal clustering centers, improves the robustness and stability of modal identification results, and realizes the automatic and accurate identification of dam modal parameters. The effectiveness and accuracy of the proposed modal parameter identification method are verified by numerical examples, and the method is applied to the vibration test of concrete dam prototype to verify its applicability in practical engineering. This study provides a reference for the vibration test analysis and structural health diagnosis of concrete dams under environmental excitation.