In this paper, thermal effect of plasma arc welding is investigated, and temperature field of ferrite stainless steel is acquired. Thermal effect of plasma arc and subsequently the generated temperature field in the work piece is the key for analysis and optimization of this welding process, which is the main goal of this paper. Finite element simulation of welding process by SIMPLEC method and ANSYS software is achieved, using FSI solver for getting stainless steel temperature field, effect of parameters variation on temperature field, and process optimization for different situations of plasma and shielding gases (Argon, Helium, and mixture of both). Lastly, the results of other papers are used to verify the correctness of this paper’s results. The temperate field results have determined the effects of welding parameters, and are used to optimize plasma welding process for improving weld quality. Optimization results for different gases indicates that due to special heat of Helium gas, there is extra potential with respect to Argon gas to narrow the plasma arc and concentrate inlet heat over stainless steel.