The influence of flux and sulfur content on YAG laser welding has been investigated, and the influencing factors and mechanism were discussed. The results show that both surface activating flux and surface active element S have fantastic effects on the YAG laser weld shape, that is to obviously increase the weld penetration and D/W ratio in various welding conditions. The mechanism is thought to be the change of surface tension temperature coefficient in weld pool, thus, the change of fluid flow paten in weld pool due to the flux and sulfur. The influence of flux and sulfur content on YAG laser welding has been investigated, and the influencing factors and mechanism were discussed. The results show that both surface activating flux and surface active element S have fantastic effects on the YAG laser weld shape, that is to obviously increase the weld penetration and D / W ratio in various welding conditions. The mechanism is thought to be the change the surface tension temperature coefficient in weld pool, thus, the change of fluid flow paten in weld pool due to the flux and sulfur.