Wind power generation is clean renewable energy, and offshore areas are ideal locations for constructing wind farms. However, research on the dynamic characteristics and response features of large-scale offshore wind turbine support systems under multi-load excitation from water, wind, and electromechanical forces remains insufficient. Taking the Donghai Bridge offshore wind farm demonstration project as the engineering background, this paper studies the dynamic characteristics of offshore wind turbine cap-pile group structures based on experimental modal analysis methods, with a focus on analyzing their flow-induced vibration response under complex environmental conditions. The results show that the first-order modal frequency of the foundation pile group is 2.83 Hz, mainly manifested as tower swaying; the second-order modal frequency is 14.83 Hz, reflecting the overall bending behavior of the cap-pile group. Tidal flow and waves have significant effects on the structural vibration response, especially the wave force energy is much greater than tidal flow pulsation, and its impact on foundation pile group vibration cannot be ignored. Therefore, this study provides important reference for the design and optimization of offshore wind turbine support systems, particularly in addressing safety and stability under multiple load scenarios.