The arc stability and droplet transfer phenomenon in welding process were analyzed with the help of a high-speed camera system, and the correlation mechanism was explored of arc voltage-arc physical phenomenon-weld morphology and microstructure-mechanical properties of joints in MIG welding of 4 mm-thick 304 stainless steel. The results show that when the voltage is 20.6 V, the welding arc heat distribution is uniform, and the droplet transfer form is a stable and continuous short-circuit transfer. The macroscopic shape of weld is good, and the microstructure is vermicular ferrite. The welded joint has high strength and plasticity. When the voltage drops to 16.6 V, the arc length becomes shorter, and the droplet has been short-circuited before it grows up. Heat conduction is slow, and undercut defects and grooves appear on the upper surface of weld. Part of lathy ferrite appears in the microstructure of weld. When the voltage increases to 24.6 V, the arc energy diverges severely, and the stability and continuity of droplet transfer decrease. Irregular fish scales appear on the surface of weld. Insufficient penetration appears on the back of weld. More lathy ferrite is formed in the lower layer of weld. When the voltage is lower and higher, the strength and plasticity of welded joint are reduced.