Under the sea-sky background, sea-sky-line detection can play an important role in detecting and tracking the ship targets. A method of sea-sky-line detection based on the analysis of the grayscale threshold is proposed. By analyzing the image and adjusting the step size of the linear filter, the gradient difference of the grayscale near the sea-sky-line is enlarged. The method based on threshold analysis is used to extract the edge coordinates, and the sea-sky-line is fitted through the least square method. The simulation verification is conducted by MATLAB as well. The results show that the proposed algorithm can accurately extract the sea-sky-line in the complex background, and has good adaptability and practicability.