Abstract: 2205 duplex stainless steel small-bore thin wall tube with φ 18 mm × 1. 5 mm was welded by tungsten inert gas welding with ER2209 filler material and 98% Ar + 2% N₂ shielding gas. Microstructure and mechanical properties of 2205 duplex stainless steel ultra-thin wall small diameter pipe welded joint were studied by means of nondestructive testing, mechanical properties test and metallographic observation. The results indicated that weld was well formed, and tensile strength of welded joint was 668 MPa, no less than that of the base metal.The fracture occurred in the heat-affected zone. The hardness of the base metal, heat affected zone and weld was 260 ～ 280 HV, which met ASTM A790 standard. The welded joint had good bending resistance. The phase boundary was smooth and there were no harmful intermetallic phases. According to ASTM E562 standard method, ferritic content in heat affected zone of welded joint was about 50% ～ 70%, which was basically balanced with austenite. The pitting corrosion resistance of welded joint met the requirement of ASTM G48 standard, and the measured maximum corrosion rate was 5. 1 mg/（dm·d）.