Abstract: A three-factor three-level orthogonal test was designed with welding current, welding speed and swing amplitude as factors. Single-track 12-layer deposited specimens of FV520B martensitic stainless steel were fabricated by gas metal arc welding （GMAW）additive manufacturing. The optimized process was determined with maximum hardness as the indicator. The microstructure and properties of the deposited specimen under the optimal process were investigated. The results show that the order of influence of the parameters on hardness of deposited specimen, from greatest to least, was: welding current, swing amplitude, welding speed. The optimal parameters were determined as the welding current of 122 A, welding speed of 5.5 mm·s⁻¹, and swing amplitude of 4 mm. Under the optimal process, the deposited specimen exhibited good forming quality without defects such as cracks, pores, or delamination. The microstructure of the deposited specimen consisted of lath martensite and minor amounts of retained austenite and δ-ferrite. From the bottom to the top of the deposited specimen, the grain size first increased then decreased, the martensite content first decreased then increased, the hardness, and strength of deposited specimen first decreased then increased and the elongation after fracture first increased and then decreased. The fracture mode of the bottom, middle and top of deposited specimens was brittle fracture, ductile fracture, and a mixed ductile-brittle mode, respectively.