Abstract: In order to illustrate the effect on liquid metal surface tension and TIG weld bead properties of Q235 steel by high silicon fly ash, some experiments were carried out. The liquid metal spreading area with and without high silicon fly ash was measured under different temperatures to establish empirical formula between temperatures and spreading area. Based on the same welding process parameters, the weld appearance, microstructure and microhardness of the joint were analyzed when the high silicon fly ash was utilized as activating flux when added into the weld. The liquid metal surface tension can be largely decreased by the activating flux of high silicon fly ash. At the same time, the deeper penetration, finer weld grain and higher mechanical properties of the joint can be obtained.