Abstract: Use two different heat input to weld T4003 ferritic stainless steel and Q450 NQR1 atmospheric corrosion resisting steel by MAG welding with austenitic welding material ER309 LSi-G wire. The properties of the two kinds welded joints were studied by means of microstructures and mechanical properties tests. The results showed that no defects are found in the two kinds of welded joints,and properties of the welded joints were good. Compared with 0. 82 k J/mm heat input,the welded joints with 0. 62 k J/mm heat input had a good impact toughness and hardness. The fusion line was narrower,the grain size was finer,and the tensile value was higher,which is more suitable for welding dissimilar steels of T4003 ferritic stainless steel and Q450 NQR1 weathering steel.