Abstract: The application of activating fluxes in all-position welding for tube sheet was studied.The activating fluxes were deposed on the metal surface of the low-carbon steel tube of 3 mm thick by a brusher without the groove preparation.According to the impact of activating fluxes on the force state of the pool in all kinds of positions and the analysis in penetration increase mechanism,the tube sheet was divided into several sections.The optimal welding parameter was obtained by the experiments.The welding joint quality met the relevant standards.By this process,it was no need for groove preparation.As a result,the demand for welding wire was decreased.Consequently,the efficiency of welding was improved substantially.The application field of activating fluxes can be extended.