Provided is an apparatus and method for detecting a horizon which is necessary to detect a camera movement when compositing sea images in a marker-free sea-image camera tracking system. In the method, an ROI is selected near a horizon in a still image of the moving image of the sea, and each maximum point corresponding to the maximum brightness difference is detected from brightness differences between two pixels of each pair having two symmetrical pixels in each column of the ROI. The horizon is detected through a line-fitting using the maximum points. Therefore, the result of horizon detection can be very stable and a horizon can be easily detected in a sea scene having hundreds of frames.