Distributed Optical Fiber Composite Overhead Ground Wire (OPGW) health monitoring based on fiber Bragg grating (FBG) sensor has been widely studied. Aiming at the OPGW cable dancing event, we propose a wave wavelength prediction method based on constructing cubic spline function fitting. Firstly, the mathematical relation between FBG sensor phase shift signal and wave is established through theoretical analysis. And then the results of phase shift signal before and after cubic spline function fitting under ideal dancing condition are analyzed by simulation, it proves the effectiveness of the proposed method. Finally, it is applied to a 220 kV line in Ili Kazak Autonomous Prefecture, we compare and analyze the intensity chart of dancing monitoring in typical weak wind environment and strong wind environment. According to the actual monitoring results, the proposed method in this paper is effective on enhancing the signal characteristics of intensity graph, and effectively estimate the waving wavelength of about 200 m in strong wind condition and 50 m in weak wind condition. This method provides technical reference for intelligent monitoring of image recognition of dancing events and has strong application prospect.