Variations in the Mn/N ratio greatly affect the formation of reformed austenite and precipitation and two-phase ratio in duplex stainless steel (DSS) during the welding thermal cycle. Therefore, investigating the influence of the Mn/N ratio on the multipass welding heat-affected zone (HAZ) microstructure is beneficial for enhancing the comprehensive mechanical properties of the HAZ of thick low-nickel-type DSS plates. This study examined the effect of the Mn/N ratio on the microstructure evolution and mechanical properties of the multipass-welded HAZ of low-nickel-type DSS in comparison with the solid solution state and 2205 DSS. The higher nitrogen content (3.77 Mn/N ratio) resulted in substantial partial transformed austenite formation with a high volume fraction of 58.7%, yielding a tensile strength of 836 MPa and an elongation of 39.5%, indicating its high strength and plasticity. As the Mn/N ratio increases to 17.80, an increase in the Widmanstätten austenite (WA) amount caused cellular Cr₂N formation at the δ/γ phase interfaces and dislocation walls in ferrite for HAZ, considerably decreasing elongation compared to the solid solution state. For DSS with a high Mn/N ratio of 65.91, the grain boundary austenite (GBA) amount initially increased and then decreased, while the intragranular austenite (IGA) and WA amounts increased in HAZ, inhibiting Cr₂N precipitation and reducing ductility and strength. Compared with the 2205 DSS, the fracture surfaces of DSS with lower Mn/N ratios exhibited brittle fracture. The combined strengthening effect of the high-nitrogen solid solution and Cr₂N precipitation on the austenite phase increased the hardness difference between the two phases in the HAZ of the 3.77 Mn/N ratio. In addition, the formation of some large (Cr, Mn) O inclusions promoted crack initiation to some extent, lowering the impact energy to 24.2 J. High Mn/N ratio addition led to more GBA and WA segmented ferrite refinement, as well as the formation of small amounts of IGA, which hindered crack propagation. Moreover, the precipitation of a small amount of the σ phase increased the hardness of ferrite, reduced the hardness difference between the two phases, and increased the impact energy to 134.8 J.