The invention relates to an automatic wielding technology of a narrow gap MAG of a hydraulic cylinder body, which is characterized by comprising the following steps: materials loading, cartridge chucking, assembly, tailstock propping tightly, trolley walking, welding gun position trimming, welding, trolley walking and baiting. The invention has the advantages that the technology greatly reduces welding line volume, saves solder wire consumption, and lowers stress damage and deformation of workpieces; as heat input is low, welding line metal flaw can be prevented, and flaw generated in the process of magnetic heat treatment can be eliminated; the welding line grain is fine and good in tenacity; and the welding technology has high cladding speed and low splash and arc blow, can obtain the welding line with good weld bead shape, good quality and beautiful appearance, does not generate noise and harmful gas, has high production rate and simple operation, and is easy to achieve mechanization and automation.