Abstract: Nitrogen loss during the welding process of high-strength nitrogen-containing austenitic stainless steel has long been a key factor limiting its application. A novel pressurized ambient metal inert-gas arc welding (MIG) method was proposed. Experiments were conducted using a high-strength nitrogen-containing austenitic stainless steel base plate with a nitrogen content of 0.64% and a high-nitrogen steel welding wire with a nitrogen content of 0.8%, and the effects of pressurized ambient MIG parameters on the nitrogen content, porosity characteristics, and comprehensive mechanical properties of the weld were systematically investigated. The results indicate that after pressurization, the mass fraction of nitrogen element in the weld increases from 0.54% to 0.60%, and the number of pores sharply decreases from 22 to 3. The yield strength of the weld increases from about 300 MPa to over 700 MPa, and the ultimate tensile strength reaches up to 924.1 MPa; however, the impact absorption energy decreases from about 32 J to about 11 J. Thermodynamic calculations verify that when the ambient pressure is 100–180 kPa, the theoretical nitrogen solubility in the weld rises from 0.81% to 1.01%, which confirms the application potential of pressurized ambient MIG; however, because the weld solidification does not pass through the single austenite phase region, the actual nitrogen content does not reach the ideal level in the thermodynamic calculation. This conclusion provides a theoretical reference for welding process optimization and industrial application.