Abstract: Objective This paper studies the plume morphology and the flow characteristics of the back molten pool during ten-thousand-watt laser scanning full-penetration welding of 8 mm thick carbon steel plates, analyzes the formation reasons of the humps on the back of the weld, aims to propose methods to improve the formation of hump weld beads, so as to achieve stable single-sided welding with back formation in ten-thousand-watt laser scanning welding. Methods Two high-speed camera systems are used to collect information of ten-thousand-watt laser scanning full-penetration welding tests on 8 mm thick carbon steel plates. The plume morphology and the flow characteristics of the back molten pool are studied, the reasons of hump formation are analyzed, and it is proposed to improve the formation of hump weld beads by means of laser scanning. Meanwhile, an attempt is made to suppress the recoil effect of the plume by using a laser scanning heat source. Through process optimization, the area and surface tension of the back molten pool are effectively increased to promote the rapid backflow of the large-sized liquid column melt. Results The research results show that, during full-penetration welding, a raised liquid column is first formed in the back molten pool of the weld under the reaction force of the keyhole steam. As the welding continues, new metal melts continuously flow into the liquid column. At the same time, under the action of the surface tension of the molten pool, the melt in the liquid column is continuously drawn out and flows back to the rear molten pool. However, when the velocity of metal flowing into the liquid column is too fast and exceeds the backflow speed, a large-sized liquid column will move backward as a whole along the backflow direction. Since the volume of this part of the metal melt is too large to return to the molten pool through surface tension, it finally cools and solidifies to form a hump. Conclusion Using a laser scanning heat source suppresses the recoil effect of the plume. The area and surface tension of the back molten pool are effectively increased through process optimization. The rapid backflow of the large-sized liquid column melt is promoted, stable back formation of single-sided welding in ten-thousand-watt laser scanning welding is successfully achieved.