Aim at the problem of sea-sky-line detection in sea-sky background image,a new method which is based on region variance and RANSAC algorithm is proposed.First,position of maximal variance in each predefined region in the image is hunt out,and coordinates of some possible sea-sky-line points are made certain according to the position,then line parameter of sea-sky-line is solved combined with RANSAC algorithm.The experimental results show that this method can detect out sea-sky-line under complicated sea-sky background effectively,and has many advantages such as strong robustness,speedy calculation and high practical value.