The line intercept histogram Otsu method has good segmentation performance, but for the weak-light part of the image, it can only divide it into background, and many details of the image are lost. Based on the analysis of the essence of the line intercept histogram algorithm, a directional fuzzy derivative Otsu method is proposed to segment the weak light part of the image. At the same time, in order to enhance algorithm universality, the segmentation results are fused with segmentation results of the traditional Otsu algorithm. Firstly, the directional fuzzy derivatives are used to replace the neighborhood mean of the pixel to achieve a good performance in segmentation of the weak-light part of the image and suppress noise. Then the segmentation results are fused with the Otsu segmentation results to get more accurate threshold segmentation results. Experimental results show that the proposed algorithm can segment the weak-light part of the image more accurately in comparison with other segmentation methods, which provides better noise reduction effects.