With the assistance of the small diameter side-blown gas, the fiber laser welding of SUS 304 stainless steel sample is performed. The behaviors of keyholes, molten pools and metallic vapor are observed by using the high-speed photography equipment under the background light source and the fusion depth and formation of welds are also analyzed. The results indicate that, when the laser power, welding speed, defocusing amount, position and angle of gas jet nozzle, and other welding parameters are constant, the fusion depth of welds increases with the increment of small diameter side-blown gas flow. The small diameter side-blown gas can open the keyholes obviously, maintain the stability of the keyholes, and also enlarge the area of the molten pool. The gas flow value has a great influence on the formation of welds. Without the small diameter side-blown gas, the uniformly-distributed weld formation with lots of the spatters is achieved. When the gas flow value is small, the uniformly-distributed weld formation with few spatters is achieved. When the gas flow value is too large, the spatters are few and the weld formation is poor.