Surface roughness measurement using the phenomenon of speckle caused by scattering of coherent or partially coherent light by rough surface was a promising online measurement technique. The speckle elongation effect of the far-field scattered field formed by the scattering of the narrowband continuous spectral beam by random rough surface and the feasibility of applying the effect to the surface roughness measurement were studied in this paper. Theoretical and simulation studies show that the speckle elongation becomes larger as the observation point moves away from the center of the scattered field; moreover, the speckle elongation increases as the surface roughness becomes smaller at the same observation position. The experimental system with Superluminescent Diode(SLD) as the light source was constructed. The surface roughness was measured by the optical roughness index which was derived from the speckle elongation. The roughness measurement experiment was carried out on the EDM surface roughness specimen. The results show that the optical roughness index monotonically decreases as the surface roughness of the measured surface increases. Surface roughness measurement system with the narrowband continuous spectral light source has a larger measurement range than a group of light sources with discrete wavelengths.