In this paper, a brief and effective method-arithmetic mean thresholding is suggested for segmenting interference image of complex light interfereometer used in automatic measurement of surface roughness. The roughness profile information extracted from binary image after thresholding and calculated results of by sampling enough height deviation data from roughness profile coinside with those charted and measured by TALYSURF 6 profilemeter. This thresholding method are also suitable for segmenting interference image of laser surface roughness interferometer.