The ship target in the long-range imaging is lack of the features such as size, texture ,shape and so on, so the ship target detection in the context of complex sky and sea has much difficulty. However, the character that the ship target is always located in the sea-sky area will help narrow down the search and suppress the unnecessary noise interference outside the area. The paper proposes a novel method of ship target detection based on sea-sky-line Detection. The method first use a robust and efficient method to extract the sea-sky-line region based on segmentation result, Then, we design a multi-scale filter and adopt an adaptive threshold method of image segmentation to extract the targets from the filtered image. Our filter is adaptive to the variable size of targets and has good ability to overcome the noise. The experimental results show that this method can detect IR target such as warship in a complex sea background.