Abstract: Full penetration welding process experiments were carried out on 6. 0 mm N6 nickel alloy by plasma arc welding. Effects of welding current, welding speed, ion gas flow rate and welding heat input on formation of weld and mechanical properties were investigated by OM, SEM and tensile testing machine. The results showed that welding parameters optimized by orthogonal design was used to obtain well formation of weld, the average tensile strength of the joint was 302 MPa, and the joint coefficient was 87. 3%. The main welding parameters had a critical influence on formation of weld and mechanical properties. There was a synergistic effect between welding current and ion gas flow. In a certain range, the weld width and penetration increased with the increase of welding current and ion gas flow, but decreased with the increase of welding speed. However, if the welding speed exceeded a certain value and the welding speed continued to increase, the back of the weld produced a “hump”. The welding heat input increased from 1. 5 kJ/mm to 2. 14 kJ/mm, the grain size in the weld area increased sharply, and the grain size decreased from 5. 9 to 2. 3, while the tensile strength of the welded joint increased first and then decreased. The tensile fracture changed from the mixed fracture mode of toughness cleavage to the brittle intergranular fracture mode.