A clutter-adaptive target-detection method is proposed to detect small moving targets in midwave infrared maritime scenarios. In particular, we focus our attention on the sea-sky background and targets of interest are ships along the horizontal sea-sky line. In the distant sea-sky background infrared imagery, small targets frequently appear as weak intensity and do not have enough structure information in the vicinity of horizontal sea-sky-line, that is sea-sky region, and the complexity of background clutters has direct impact on the target detection performance. Thus, a fuzzy system constructed by the extracted image-based features is designed for the clutter classification. And then, based on the determined clutter type, low clutter or high clutter, the horizontal sea-sky line can be detected successfully. In the target detection stage, under the guidance of the sea-sky line, a modified multilevel filter method is applied to enhance the target in the sea-sky region. Finally, the recursive segmentation method and the target validation technique are adopted for the target extraction and validation. In the experiments, the target detection performance of proposed method is validated by extensive experiments on infrared images taken in different imaging conditions. It achieves high accuracy with a low false alarm rate.