Aiming at the problem of low accuracy of ship target detection at sea， a lightweight ship target detection algorithm YOLOShip was proposed on the basis of the improved YOLOv5. Firstly， dilated convolution and channel attention were introduced into Spatial Pyramid Pooling-Fast （SPPF） module， which integrated spatial feature details of different scales， strengthened semantic information， and improved the model’s ability to distinguish foreground and background. Secondly， coordinate attention and lightweight mixed depthwise convolution were introduced into Feature Pyramid Network （FPN） and Path Aggregation Network （PAN） structures to strengthen important features in the network， obtain features with more detailed information， and improve model detection ability and positioning precision. Thirdly， considering the uneven distribution and relatively small scale changes of targets in the dataset， the model performance was further improved while the model was simplified by modifying the anchors and decreasing the number of detection heads. Finally， a more flexible Polynomial Loss （PolyLoss） was introduced to optimize Binary Cross Entropy Loss （BCE Loss） to improve the model convergence speed and model precision. Experimental results show that on dataset SeaShips， in comparison with YOLOv5s， YOLOShip has the Precision， Recall， mAP@0.5 and mAP@0.5：0.95 increased by 4.2， 5.7， 4.6 and 8.5 percentage points. Thus， by using the proposed algorithm， better detection precision can be obtained while meeting the requirements of detection speed， effectively achieving high-speed and high-precision ship detection.