Aimed at the problem of sea-sky-line detection in complicated sea-sky background image, a novel method, based on phase grouping and gray statistics, was presented. Firstly, a lot of line segments were obtained by using phase grouping method on edge image. Secondly, the line segments were classified according to two parameters, the slope angle and the distance to image origin. Several classes which have the largest sum of length of the line segments were selected, and edge points in each selected class were used to fit a straight line, thus several candidate sea-sky-lines could be gained. Finally, the correct sea-sky-line could be elected through comparing statistical gray values of the regions which correspond to the same horizontal pixel coordinates and take the points on the candidate sea-sky-lines as center points. Many experimental results show that the proposed method can detect the sea-sky-line under complicated sea-sky background effectively and has strong robustness.