Abstract: In order to explore the way to reduce energy threshold in X-shape welding formation of TC4 sheet, the influence of surfactant type, coating thickness and coating position on back reflection induced synergistic laser welding was systematically analyzed in this paper. The results show that the effect of different kinds of surfactant were different. Among them, coating NaCl and NaF is conducive to the formation of back reflection plume effects, while coating TiO₂ has some weak effect. At the same time, the back reflection plume effect was further enhanced the increase of the surfactant coating thickness NaF was more sensitive to the coating thickness than NaCl. In addition, there is little effect when coating surfactant on the back of weld. It is concluded that the effect of surfactant on the back reflection induced synergistic laser welding of TC4 sheet is mainly due to the gain of laser energy absorption.