The behaviors of YAG laser welding process of ferritic stainless steel with activating fluxes were investigated in this study. Some conventional oxides, halides and carbonates were applied in laser welding. The results showed that the effect of oxides on the penetration depth was more remarkable. Most activating fluxes improved the penetration more effectively at low power than that at high power. The uniform design was adopted to arrange the formula of multicomponent activating fluxes, showing that the optimal formula can make the penetration depth up to 2.23 times as large as that without flux, including 50% ZrO2, 12.09% CaCO3, 10.43% CaO and 27.48% MgO. Through the high-speed photographs of welding process, CaF2 can minimize the plasma volume but slightly improve the penetration capability. The behaviors of YAG laser welding process of ferritic stainless steel with activating fluxes were investigated in this study. Some conventional oxides, halides and carbonates were applied in laser welding. activating fluxes improved the penetration more effectively at low power than that at high power. The uniform design was adopted to arrange the formula of multicomponent activating fluxes, showing that the optimal formula can make the penetration depth up to 2.23 times as large as that without flux including 50% ZrO2, 12.09% CaCO3, 10.43% CaO and 27.48% MgO. The high-speed photographs of welding process, CaF2 can minimize the plasma volume but slightly improve the penetration capability.