Abstract: In this paper, a bypass-coupled triple-wire indirect arc additive manufacturing process is adopted to simultaneously melt two dissimilar wires, namely ER50-6 low-alloy steel wire and ER316L stainless steel wire, to fabricate single-track multi-layer wall specimens of high-strength low-alloy steel. Systematic experiments are carried out to investigate the arc morphology and metal transfer mode during the synchronized feeding of three wires. Welding parameters are monitored and adjusted via an automatic wire feeding system to optimize the forming accuracy and mechanical performance of deposits, and the strengthening mechanism of mechanical properties for additively manufactured components is analyzed.The experimental results indicate that the optimal forming performance with smooth wall surfaces is achieved under the following parameter combination: welding current of 320 A, layer height of 3 mm, main wire feeding speed of 6.5 m/min, side wire feeding speed of 5 m/min and travel speed of 8 mm/s. Under these conditions, the metal deposition efficiency reaches 11.4 kg/h. During the rapid cooling of the molten pool, supercooled transformation of austenite takes place, accompanied by martensitic and bainitic transformations. The composite microstructure synergistically improves the comprehensive performance of components.The average ultimate tensile strength of specimens is 1035 MPa with an elongation of 26%, and the average micro Vickers hardness is 380 HV. All mechanical properties meet the acceptance criteria for high-strength low-alloy steel. This proposed process provides a novel and feasible technical route for high-efficiency, high-performance arc additive manufacturing of HSLA steel.