Abstract: Composite plates have found widespread application in industrial manufacturing due to their superior properties. The welding process directly impacts the service life and performance of these composite plates. This study focuses on composite plates made of 304 stainless steel and Q345R steel, utilizing Metal Active Gas (MAG) welding technology to systematically evaluate the welded joints. Through the selection of materials, the setting of welding process parameters, post-weld heat treatment, and non-destructive testing, the quality and performance of the welded joints were assessed. Results indicate that appropriate selection of welding materials and process parameters is crucial for the welding quality of composite plates. Post-weld heat treatment significantly reduces the residual stress in the welded joints and improves the microstructure of the welded joints.