Abstract: With a new developed misplacing and synchronizing double-sided TIG backing welding technique,the characteristics of welding pool formation are invesitgated by means of monitoring visual images.The experiment results show that presetting welding gap is an effective approach to ensure the root fusion,which makes it possible to weld large and thick plates without back chipping.The pulsed TIG welding is an optimal choice for backing welding of large and thick plates.Both sides of the groove root melt during the peak current period due to the strong melting ability of the arc.The welding pool rapidly solidifies during the base current period,preventing the dropping of welding pool.During misplacing and synchronizing double-sided backing welding,the rear arc leads to an increment of the high temperature area in the workpiece and elongation of the front pool.The macrograph of the weld shows that the base metal sufficiently melts and the lateral wall fuses well.The weld surface is smooth with no pores,cracks and inclusions.