Abstract: A new method to detect the weld seam centerline by calculating the offset-distance based on welding colorimetry image was proposed.The concept of offset-distance was defined and the offset-distance arithmetic that analyzes the welding colorimetry image was applied to obtain the deviation between the weld seam centerline and the arc column center.Based on the offset-distance formula,the relationship of some factors such as the signal intensity of CCD,welding temperature field,center wavelength of a narrow-band filter,emissivity of welding region,weld seam and the radiant intensity was obtained.Model giving the relation between the offset-distance of welding colorimetry image and the discrete numerical welding temperature field and the deviation between the weld seam centerline and the arc column center had been established.The numerical solution of the model had been presented and the relation curve had been shown.The conclusion indicates that the output parameter of the model which is the offset-distance of the welding colorimetry image and the input parameter which is the weld seam centerline has a good linear relation.Therefore the deviation between the weld seam centerline and the weld arc column center which is hardly detected directly from the welding visual image can be calculated by the offset-distance which is easily obtained from the welding colorimetry image.The factors influenced the accuracy of the model have been analyzed.The model makes the application of the offset-distance of welding colorimetry image for detecting the actual weld seam centerline more practical and simpler.