Under different protection conditions of shielding gases composed of pure N2, pure Ar, or N2 /Ar mixing, one experiment of fiber laser welding of 304 austenitic thin plate with a thickness of 2 mm is conducted, and the effects of shielding gas pressure, gas type, and mixing ratio on the weld joint morphology and mechanical property are investigated. The results indicate that the larger the shielding gas pressure is, the larger the weld penetration depth becomes, but the gas pressure has no obvious influence on the weld strength and hardness. When the N2/Ar mixing gas is used as the shielding gas, the larger the N2 content ratio is, the larger the penetration depth becomes, and the tee-heeded weld is kept no matter what the content ratio of shielding gas is. The type and content ratio of shielding gas have no obvious influence on the weld strength and hardness, but the strength and hardness are both larger than those of substrate.