Abstract: The penetration depth and width of the weld determine the popularity of the application of low carbon steel. Controlling the welding speed, changing the defocus amount, introducing protective gas and adjusting the welding attitude can improve the welding effect, but usually require high power laser. In this paper, the weld penetration and the grain phase morphology of the weld are improved by introducing active flux at low power. By introducing single welding additives titanium dioxide and magnesium oxide, the weld penetration depth increased from 2354 μm to 2838 μm and 2745 μm, adjusting the mass fraction of titanium dioxide and magnesium oxide in the composite additives, the weld penetration depth was further improved, and the maximum penetration depth reached 3361 μm, which was more than 42% higher than the weld penetration depth without welding additives. The improved welding penetration is attributed to the synergistic action of the two welding AIDS. This paper provides a general method for improving weld penetration under low power.