Abstract: QN1804L ultra-low carbon CrMnN austenitic stainless steel was welded by plasma arc welding, and the welded joints were treated by thermomechanical treatment and solid solution treatment, respectively.The results show that the microstructure of the joint in welded state is composed of austenite and ferrite, the ferrite is distributed between austenite grains in skeleton and strip shape, and the hardness of the weld is slightly higher than that of base metal. After thermomechanical treatment, the weld microstructure is distorted, the grains are elongated, and the hardness of the weld is much higher than that of the base metal. At the same time, due to deformation strengthening, the weld strength is also significantly improved.After solid solution treatment, the ferrite in the weld transforms into austenite at high temperature, and those phase not transformed in time are distributed in spot, and the hardness of the weld is basically consistent with that of the base metal.The welds after different heat treatment have good corrosion resistance.