Ocean data is a multi-scale dynamic signal representation,with strong instability and non-linearity.As a new development time-frequency analysis method,Hilbert-Huang transform (HHT),by virtue of well behavior in nonlinear and instability analysis,plays an important role in many fields.The end effect is the difficult problem in the HHT.In the cubic spline,“flying wing” phenomenon will pollute the internal data,resulting in a large amount of data distortion.Therefore,we propose a new approach which is called five-points-symmetry extending.Simulation results show that improved HHT method has better performance in analyzing harmonics to traditional HHT method which provides theoretical and practical significance for the wave signal processing.