Abstract: The chaotic dynamics of skywave over-the-horizon rader (OTHR) sea clutter is confirmed by using the Cao method for phase space reconstruction, calculating the maximum Lyapunov exponent by Rosenstein method and Kolmogorov entropy by an improved Grassberger-Procaccia algorithm (GPA) method and the characteristics of forecasts by RBF neural network of real OTHR sea clutter.Simulations show that the strange attractor of OTHR sea clutter has stable convergence correlation dimension, positive maximum Lyapunov exponent and positive Kolmogorov entropy, and the characteristics of short-term forecasts but long-term prediction, which clearly proves that the HF sea clutter is generated by a low dimension chaotic system.Then, apreliminary discussion of the influence caused by ionosphere on chaotic characteristics of OTHR sea clutter is conducted, which indicats that the ionosphere has a notable impact on the chaotic characteristics of OTHR sea clutter.The conclusions above are applicable in the research of modeling for HF sea clutter and target detection within HF sea-clutter background.