Usually roughness reflects the machining quality of the machined surface, but when the roughness of different surfaces is the same or similar, it is not sufficient to accurately distinguish the tool marks and grain concavity and convexity relying only on roughness. Therefore, the yard stick method is a better way to calculate the fractal dimension of the surface. In addition, in order to accurately reflect the grain concavity and convexity, the method of filtering can be used to weaken the influence of tool marks. The results show that the fractal dimension after filtering can reflect the features of grain concavity and convexity quantitatively.