Abstract: Utilizing the silicon photoelectric transistor as a sensor, the authors developed an automatic penetration control system for MIG welding. Thus the root pass with back bead formation was welded successfully. In this paper the penetration performance of MIG welding pool was discussed. A dual-threshold controller based on pulsed wire feeding was discribed in detail. This was designed for the fast adjusting of the arc energy in MIG welding process. The system developed can be applied to the auto-welding of root passes on presure vessels, etc.