The present invention discloses an image binarization threshold determination method in sea-sky-line detection. The method comprises the following steps: image blocking: dividing an image into upper, middle and lower strip-like sub-images in the height direction, respectively corresponding to a sky region, a sea-sky-line region and a sea surface level; histogram statistical calculation: performing histogram statistics on the sub-images of the sky region and the sea surface region, and separately obtaining the largest gray-scale value Ttop and Tbot of the two regions, and then comparing a numeric value of the Ttop with a numeric value of the Tbot, and storing the larger numeric value into the Ttop, and storing the smaller numeric value into the Tbot; and binarization image threshold determination: setting a threshold sequence in three cases according to a difference value of the Ttop and the Tbot, and individually calling a sea-sky-line detection program to try detecting a sea-sky-line. The method has relatively high adaptability to different environments, and is of significance for sea-sky monitoring tools and carrier posture measurement.