Abstract: Closed-loop control of full penetration is an attcntion-focussed problem in the field of welding because it is very difficult to measure the state of full penetration from the top side of the weld.In this contribution,it is discovered that bascd on strict experiments and statistic analysis there is a close relationship between the backsidce weld width and the average top surfacc depression depth (weld depression cross-sectional area divided by weld width).This discovery provides a basis for the authors presentation of a new control strategy for full penetration where the real time measurement of the state of full penetration is done by a structured-light 3D vision sensor.