Abstract: Arithmetic of welding visual image centroid is studied to detect weld seam center indirectly by calculating centroid of welding visual image obtained from welding visual system in real-time welding instead of direct image processing.From several equations including the relation of response voltage of CCD(charge coupled device) and welding temperature field,wavelength of a narrow-band filter,optical parameters,emissivity of workpieces'surface and weld seam,radiant intensity and so on,the law of Max Planck,the equation between the response voltage of CCD and the gray value of welding visual image and the formula of welding visual image centroid,the model indicating the relation between centroid of welding visual image and temperature of welding pool and heat affected zone has been deduced.The model reveals the relation between the centroid of welding visual image and weld seam center.The tendency of centroid changing with weld seam center has been shown by diagram after welding temperature field simulated.The characteristics of centroid model wone analysed.The centroid of welding visual image has been a closely relevant with weld seam center,which make the application of centroid of welding visual image more reasonable and promising in welding seam tracking.