Welding characteristics of laser-arc double-sided welding (LADSW) for aluminum alloy are studied. Arc physical characteristics are investigated by spectroscopic diagnosis, arc voltage measurement and estimation of electron density by Stark broadening method. With the preheating of tungsten inert gas (TIG) arc, the results show that the weld appearance of laser welding in LADSW is improved, and porosity in weld can be drastically reduced and even eliminated. The shape of weld cross-section can be controlled. Due to the symmetrical weld cross-section and less weld defect inside, the joint of the symmetrical "X" cross-section has the best mechanical properties. The tensile strength is more than 90% of the base metal. Compared with conventional TIG arc, the typical arc shape of LADSW is compressed. There are more metal atoms and less Ar atoms in LADSW arc than those of conventional TIG arc, and the arc voltage is decreased and more stable. The electron density of LADSW is higher than that of conventional TIG arc. The plasma area of laser welding is increased in LADSW compared with conventional laser welding under the same condition.