Offshore wind energy has a broad prospect of application as a green pollution-free clean energy to ease global warming.The offshore wind turbine structures are the basic facilities for offshore wind energy exploitation.During the service life,damages of the foundation structures are unavoidable as a result of the action of various loads including operational and environmental forces.The damage may result in a significant change in the modal properties of the structure,such as natural frequencies and mode shapes. Thus,the modal parameter estimation,which is the process of determining modal parameters from response data,of an offshore wind turbine structure,is the essential step for structure health monitoring (SHM).The offshore wind turbine structures are subject to ambient excitation from sources such as wind,waves etc.The input ambient excitation is often unmeasurable.Therefore,the modal parameters of these structures must be identified by output-only modal identification methods.In this paper,modal parameter identification for offshore wind turbine is studied.A time-domain modal identification method which combined Natural Excitation Technique(NExT)and Polyreference Complex Exponential(PRCE) method(called NExT/PRCE method for short)is applied to identify modal parameters under ambient excitation. Numerical example applied a tripod-type offshore wind turbine structure.Considering the operational condition of the structure,the dynamic analysis was performed under simulated random wave load and wind turbine load.The modal frequencies and corresponding mode shapes were identified from the obtained time histories of response signals.Stability diagram and MAC value were set to eliminate the false modes.And the influence of the chosen reference channel on modal parameter identification was also studied. The results are in good agreement with finite element model,which validates the proposed method.