Abstract: In this paper, a penetration control syslcm for MIG/MAG welding was developed. With CCD (Charge Coupled Device) as the sensor, the tempera lure distribution across the back of weldbcad can be detected without calibration so that the penetration degree of weldbcad may be recognized. A nonlinear unit was used to improve the stability and response of the control system. Then a process cf welding penetration control was realized precisely.