The invention discloses an automatic welding device and an automatic welding method for a thin-shell car body. The device comprises a portal-frame welding robot, elevation-type head-tail frame positioners which is arranged on the side of the welding robot, a reversing frame which is installed between the positioners and a control mechanism which is used for controlling the welding robot and the positioners to work. The method comprises the following steps of back welding a weld seam which is more than 1mm, then filling the weld seam, placing the top/left side/bottom/head/right side/tail part of a car body on a horizontal position by rotating the elevation-type head-tail frame positioner, and welding the corresponding weld seam. The automatic welding device has the beneficial effects that automatic welding of different weld seams in different forms of different car types can be satisfied, the continuous welding of a majority of weld seams outside the car body can be realized, and the production efficiency is greatly improved; by adopting the welding method, not only can the quality of the weld seam be improved, but also the welding speed is high, the heat input is small, the weld deformation is small, and the subsequent assembling can be guaranteed.