To accurately measure the crack size on the surface of nuclear fuel rods in a complex underwater environment, a method based on linear structured light is proposed. First, a two-dimensional grayscale search is performed on the acquired image to select the area to be processed, and then to further determine the crack area by horizontal projection and first-order differentiation. Then, threshold division of the crack area is performed to reduce the range of this area, and the upper and lower boundaries of crack are determined using the seed point judgment method. Finally, the least squares method is used to fit the centerline of structured light, the intersection point calculated by the centerline equation and the crack boundary point set is used to determine the specific location of crack, and then, the crack boundary point is converted into three-dimensional coordinates for distance sampling to calculate the actual size of the crack. Experiments demonstrate that the proposed method can accurately and quickly measure the crack size of a fuel rod with an error within 0.03mm, making this method appealing and convenient.