The invention discloses an automatic plate welding device. The automatic plate welding device comprises a rack, an argon arc welding machine and a control cabinet. An argon arc welding gun of the argon arc welding machine is erected on the rack and is controlled to operate through the control cabinet. A transverse guide block is mounted on the rack. One end of the transverse guide block is provided with a transverse motor. The transverse motor drives a transverse sliding block to slide on the transverse guide block. A longitudinal sliding block is fixed to the transverse sliding block. The rear end of the longitudinal sliding block is provided with a longitudinal motor. The longitudinal motor drives a longitudinal guide block to move on the longitudinal sliding block. A vertical cylinder is mounted on the longitudinal guide block and drives a rotating motor to move up and down. The rotating motor drives the argon arc welding gun to rotate so as to adjust the welding direction. The automatic plate welding device is simple in principle, convenient to use, low in cost and capable of being widely applied to welding of small and medium-sized plate products.