Abstract: A passive vision sensing system for the image of twin-wire MIG welding molten pool of aluminum alloy has been set up.Near-infrared CCD and compound filters system are used to eliminate the arc disturbance, and the best filter window of aluminum alloy is obtained, from which good images can be acquired.By using image enhancement, threshold partition, boundary extraction, the molten pool boundary and the molten pool geometric parameters are extracted through image processing.