To solve the problem of detecting white foreign fibers in cotton, the optical characteristics of cotton and foreign fibers were analyzed, and then a detecting method based on a line-construction laser for illumination was set up. The experimental results indicated that the histograms of the gray images of cotton and white foreign fibers were of dual-peak distributions, and the contrast between white foreign fibers and cotton was remarkably enhanced by the detecting method. Thus, a simple threshold segmentation algorithm was effectively used for detection of the white foreign fibers in cotton. The results indicated that the mean detectable rate of six typical foreign fibers reached to 92.08%.