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（welding current: 180～200A, arc voltage: 22～24V, welding speed: 10～12cm/min）, post-weld heat treatment, and non-destructive testing, the quality and performance of the welded joints were assessed. The base layer welding material was chosen as solid ER50-6 wire, the transition layer welding material as solid ER309 wire, and the cladding layer welding material as solid ER308 wire. 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. The tensile strength of the welded joints reached 519～521MPa, the yield strength reached 359～374MPa, and the elongation after fracture reached 28.5%～30.0%.