Maritime ships detection is one of the main tasks in unmanned surface vehicle (USV)'s visual system. This paper proposes a kind of USV maritime ships detection algorithm based on object proposals. Firstly, a modified edge boxes algorithm is utilized to extract the edge information of the image, and an objectness score function is established to obtain object proposals. Secondly, a histogram of oriented gradient (HOG) feature model is built for the ship, and the support vector machine (SVM) is utilized to iteratively train a classifier by a bootstrap method. Finally, the feature descriptor of object proposals is fed into the classifier, and detecting the ship. In addition, the sea-sky line is utilized to further improve the detection performance of the algorithm based on the environment of USV. The experimental results show that the algorithm can rapidly and accurately detect the ship on the sea, and achieve a relatively high detection rate. And the algorithm has strong robustness to the change of the scale and the illumination conditions.