Abstract: Heavy-lift launch vehicle's fuel tank requires aluminum alloy to be vertically assembled and welded, horizontal positions TIG arc welding is one of the methods which meets the requirements of the manufacture. The characteristic of the horizontal position weld and its welding defects were analyzed. The influence of welding current, welding speed and welding torch angle on the offset distance in weld top side was studied by bead on plate welding experiments. Moreover, the effect of welding current frequency on welding porosity defect was investigated. Research results indicate that smaller welding current and faster welding speed are beneficial for the symmetry of horizontal joints. The arc force can restrain the downward flow of the molten metal in weld pool. The welding porosity defect was the least when welding current frequency was 100 Hz. The control method for welding defects was proposed based on the above results and a good horizontal weld could be obtained by using the proposed control method.