In one-side welding with full penetration, the use of backing strips helps support molten weld metals, but increases manufacturing cycle time and cost. The backing-free welding technology is under investigation for medium-thickness structural components in the aerospace industry. Its crucial challenge is ensuring complete joint penetration and maintaining a suitable and stable backside bead width. This paper presents an innovative method of controlling weld formation based on weld-face vision sensing in arc welding. A standard laser triangulation setup is used to obtain images of weld pool, and a new weld-face feature ‘depression degree of the weld pool’ is proposed. An image processing procedure is developed to extract the depression degree of the weld pool from images in real-time. The depression degree of the weld pool is found to correlate closely with the backside bead width, and is a better indicator of the backside bead width than the weld pool width, with a higher consistency and a smaller time lag. Employing the depression degree as a feedback signal, a backside bead width control method is developed and implemented. Welding experiments on 6-mm-thick aluminum alloy 2219 plates show that the presented method is remarkably effective in achieving full penetration and keeping backside bead width constant.