In order to study the influence of side assist gas on the microstructure and the corrosion resistance of welds of austenitic stainless steels in the process of fiber laser welding, a fluid model of side assist gas and metal vapor is established. The influences of different kinds of side assist gases and their flow rates on the metal vapor temperature field and velocity vector are analyzed, and a method to increase the nitrogen content in the weld metals by means of side assist gas is proposed. The experimental results show that, on the premise of keeping the weld formation unaffected, the proposed method can effectively improve the nitrogen content in the upper layer of weld metals, restrain the ratio of ferrite in the weld metals, and enhance the pitting corrosion resistance of the austenitic stainless steel.