Due to the complexity in the operational environment of the offshore wind power structure, the accurate identification of the structure vibration source and the study on the vibration source characteristics have become the focus on the security assessment of the structure under the operation state. A new structural configuration called composite bucket foundation was applied to a certain experimental prototype of the offshore wind power structure and the main vibration source of that structure has been identified and analyzed comprehensively under sorts of capacity condition based on the vibration response data in different location of the structure obtained from prototype test. Firstly, the main vibration source type, which led to the structural vibration of offshore wind turbine, and the frequency domain properties of corresponding vibration responses were identified and made clear based on the spectral kurtosis method (SK). Secondly, the frequency domain and frequency scale on the different structural vibration source were showed in the signal components decomposed from the measured signal using empirical mode decomposition method (EMD). Simultaneously the energy proportions of the various vibration source compared to the total signal energy were counted and analyzed by vibration energy method and the result gives expression to regular pattern on the variation of main vibration source of entire structure and the distribution of relevant vibration energy with the change of operational factors. It is conclusion that the vibration sources of the offshore wind power structure under the operational conditions will change in accordance with such regularity that it begins with single environmental excitation and transforms from the simultaneous effect combined ambient loads with impeller rotation to the influence entirely coming from harmonic excitation.