Disclosed are an automatic welding system and method for large structural parts based on hybrid robots and 3D vision. The system comprises a hybrid robot system composed of a mobile robot and an MDOF robot, a 3D vision system, and a welding system used for welding. The rough positioning technique based on a mobile platform and the accurate recognition and positioning technique based on high-accuracy 3D vision are combined, so the working range of the MDOF robot in the XYZ directions is expanded, and flexible welding of large structural parts is realized. The invention adopts 3D vision, thus having better error tolerance and lower requirements for the machining accuracy of workpieces, positioning accuracy of mobile robots and placement accuracy of the workpieces; and the cost is reduced, the flexibility is improved, the working range is expanded, labor is saved, production efficiency is improved, and welding quality is improved.