The invention relates to a crossing bridge and offshore ship joint detection method in an onboard remote sensing image. The crossing bridge and offshore ship joint detection method comprises the steps of firstly, carrying out otsu self-adaptation threshold segmentation on the onboard remote sensing image, then sequentially carrying out morphology closed operation and morphology open operation on a segmentation result image, obtaining a water body district image, then carrying out binaryzation operation on the water body district image, obtaining a water body district binary image, separating target candidate districts and water body background districts, calculating the area average value of the target candidate districts, checking whether the water body background districts exist around the target candidate district or not, if the value of the area of one target candidate area is larger than the average value and is surrounded by the water body background districts, determining that the target is an offshore ship, and if the value of the area of one target candidate district is smaller than the average value and only two opposite sides of the target candidate district are the water body background districts, determining that the target is a crossing bridge. The crossing bridge and offshore ship joint detection method is high in calculating efficiency, real-time processing can be easily carried out on an onboard computer, and the detection false alarm rate is low.