FBTIG(Flux Bounded TIG)welding experiments for aluminum alloy were made using five species of single-component activating fluxes, including SiO2、TiO2、CaF2、MgO, and NaCl, which was carried out to investigate the effects of flux gap and activating fluxes on weld formation. The mechanism of increasing welding penetration was analyzed linked with the change of arc shape in FBTIG. The experimental results indicate that five species of single-component activating fluxes can increase weld penetration obviously. Specifically, SiO2 can increase weld penetration remarkably. When flux gap is 4mm, the D/W (depth/width) ratio of weld for FBTIG welding reaches 0.59. The significant increase of weld penetration is the result of comprehensive effect by arc contraction and reversal of the surface tension gradient in the liquid metal.