Abstract: A passive vision sensing system for taking the image of MAG welding molten pool has been set up.Near-infrared CCD and compound filters system composed of 1064 nm narrowband filer and 0.1%neutral dimmer film are used to eliminate the arc light disturbance and a lot of clear images are obtained. Imageprocessing software is used to extract the characteristic information which reflects the welding quality.For the single image, the gray character is analyzed and three equivalent contour lines are obtained.For continuous multi-frame images, the image aberration shade detection and mean gray distribution are put forward to study the process of MAG welding.