The invention discloses an image segmentation method and device and an image segmentation method and device applied to slag pieces. The method comprises the steps that a grayscale average of a neighborhood of each pixel point is calculated based on a grayscale value of each pixel point in an image of a to-be-tested object; a two-dimensional histogram of the image is constructed according to the calculated grayscale value of each pixel point and the grayscale average of the neighborhood of each pixel point; an optimal segmentation threshold value is calculated according to the two-dimensional histogram of the image; the to-be-tested object in the image is segmented based on the optimal segmentation threshold value to obtain an initial segmented binary image; and the to-be-tested object in the initial segmented binary image is segmented precisely through a watershed method. Through the image segmentation method and device and the image segmentation method and device applied to the slag pieces, the image can be segmented precisely.