Abstract: The influence of complex composition activating flux on the weld penetration in light beam welding was investigated, and the mechanism was discussed.The results show that the composition of flux has the largest influence on the penetration increment, and the optimal complex composition is found.Penetration increment obtained with this optimal composition flux is larger than that with single composition fluxes.The peak temperature of certain point near the heat affected zone can be measured according to the thermal curve monitoring system.When workpiece is not melted, the peak temperature of the point can be significantly increased in welding with fluxes, which implies that the fluxes can increase light absorption.And when workpiece is melted, the penetration and peak temperature varied in different trends with increasing fluxes amount, which implies that the fluxes can change the fluid flow pattern of the molten pool.In activating light beam welding, these two mechanisms cooperated and increased the weld penetration.