The invention discloses an automatic welding device for a construction steel structure. The automatic welding device is characterized in that a welding robot is arranged above a welding positioner through a hanging device, and the welding positioner is composed of multiple supporting frames and multiple upenders; each upender is composed of a track, a rack and a lifting trolley, wherein the lifting trolley is a trolley with a lifting mechanism; the hanging support is composed of stand columns, a track beam, a cantilever beam and a vertical guide rod; the welding robot is arranged on the vertical guide rod in an inverted mode and does three-dimensional space motion on the hanging device along with the vertical guide rod to automatically weld all-directional welding seams of the steel structure. Compared with the prior art, the automatic welding device is simple in structure, fast to assemble and disassemble, accurate in welding control, high in welding seam quality and working efficiency, and particularly suitable for automatically welding steel structural members such as columns with H-type and box-type column body sections, cross-shaped columns and cross-shaped beams.