The invention relates to a water sky line detection method based on gradient saliency. The method comprises steps as follows: a frame image is acquired by an optical imaging instrument, and if the image is a color image acquired by an ordinary camera, the image is subjected to standardization processing, and a 24-bit RGB (red green blue) color image is obtained; if the image is a gray image acquired by an infrared imaging instrument, the image is subjected to standardization processing, and an 8-bit gray image is obtained; the obtained standardized image is subjected to Gaussian downsampling and the like. A gradient magnitude matrix and a gradient direction matrix of the image are calculated respectively according to the type of the image acquired by the optical imaging instrument, all gradient information of an original image is reflected in the result, and accuracy of the water sky line detection result is guaranteed. Line segment detection based on a region growing algorithm is performed sequentially from high to low according to the gradient saliency, and the problem of severe noise interference due to the fact that gradient information is directly used for detection is solved.