Abstract: The high pressure deaerator of 300 MW sub-critical power plant boiler is made of SA516 GR70/SA240 TP304 composite steel plate material. The phenomenon of intergranular corrosion occurred in the welding process of composite plate by SAW +SMAW and SMAW +CO₂, respectively, which caused the deterioration of heat transfer during the work of deaerator.Aiming at the weldability of the stainless steel composite material, the technical points in welding was analyzed, and the GTAW +SMAW welding process plan was determined. 309-16 austenitic stainless steel welding material was used as the transition layer, and the welding line energy was controlled with reasonable welding groove form and process parameters, so as to reduce the dilution and burning of alloy elements in the weld. RT non-destructive test and mechanical tensile test of the weld were carried out, scanning electron microscopy（SEM）, energy spectrum（EDS） and blue dot test were used to analyze and study the microstructure, element diffusion and intergranular corrosion of the weld. The results show that: the composite weld has excellent mechanical properties, with the tensile strength of 727 MPa, yield strength of 321 MPa and elongation after fracture of 57.6%, respectively. The chemical composition and alloying elements of the dissimilar metal fusion interface are evenly distributed, and the crystal structure of the weld has good fusion properties and intergranular corrosion resistance, which can solve the technical problems of heat transfer deterioration caused by intergranular corrosion in the welding of composite plate.