The work aims to study the effects of shield gas composition for MAG welding on intergranular corrosion resistance and electrochemical pitting resistance of SUS304 welding joints. Corrosion resistance of SUS304 MAG welding joints was studied in 97%Ar+3%O2 and 95%Ar+5%CO2 welding gases were investigated in the method of sulfuric acid-ferric sulfate intergranular corrosion test for stainless steel (GB/T 4334.2—2000) and the method of pitting potential measurement for stainless steel (GB/T 17899—1999). Mean corrosion rate of welding seams on SUS304MAG welding test-piece in 97%Ar+3%O2 wielding gases was 2.78 g/(m2?h) while that in 95%Ar+5%CO2 was 2.50 g/(m2?h). After SEM scanning, MAG welding test-piece in sensitization zone in HAZ (heat affected zone) of 95%Ar+5%CO2 welding gases sustained worse grain boundary corrosion. However, the test-piece in un-sensitization zone in HAZ (between sensitization zone and seam) and weld zone of 97%Ar+3%O2 welding gases was corroded more seriously. E‘b100 pitting potential in welded HAZ and seam of 97%Ar+3%O2 welding gases was 0.208 V and 0.144 V, respectively; and that in welded HAZ and seam of 95%Ar+5%CO2 welding gases was 0.199 V and 0.155 V, respectively. The MAG welding seam test-piece in HAZ of 97%Ar+3%O2 welding gases exhibited better pitting corrosion resistance. The results shows that: in two different wielding gases, MAG welding joints show similar pitting resistance, and MAG welding joints in 95%Ar+5%CO2 wielding gases exhibit better corrosion resistance.