The invention discloses an automatic welding tool for a trailer accessory and belongs to the field of automatic industrial equipment. The automatic welding tool is composed of an air cylinder, guide rods, an air cylinder installation plate, an air cylinder top plate, a first sliding block connection plate, a first sliding block, a guide rail, a second sliding block connecting plate, a second sliding block, a bottom plate, a third sliding block, a rack sliding block, a gear, a gear linkage rod, a steel ball, sliding block front top plates, a fourth sliding block, inter-sliding-block connecting plates, a fifth sliding block and a sixth sliding block. The guide rail is installed on the bottom plate, and the six sliding blocks are installed on the guide rail. The inter-sliding-block connecting plates are installed between the first sliding block and the second sliding block and between the fourth sliding block and the fifth sliding block respectively, the sliding block front top plates are installed before the second sliding block, the fourth sliding block and the sixth sliding block, the guide rods are fixedly installed an the air cylinder top plate, and the air cylinder top plate is fixedly connected with the first sliding block, the fifth sliding block and the sixth sliding block through the first sliding block connecting plate, the second sliding block connecting plate is fixedly connected between the first sliding block and the third sliding block, the rack sliding block is connected to the front of the third sliding block, and the gear linkage rod is fixedly arranged on the gear. According to the automatic welding tool for the trailer accessory, multi-point positioning and clamping are achieved on a workpiece simply through one air cylinder.