In the process of autogenously welding of SUS 304 stainless steel plate with 12-mm thickness using high power fiber laser, because the welding pool can not maintain homeostasis in the combined action of gravity, surface tension and vapor recoil, the flow of the liquid metal in the molten pool is drastic and the poor formation is easy to cause welding defects of its surface such as spatters, surface undercut and root hump. With certain welding power and top protection gas with certain flow rate, the influences of the welding speeding, defocus, butt joint gap and the flow rate of bottom shielding gas on welding quality by changing the single variable are discussed. The results show that adopting some reasonable technology measures can efficiently improve the welding quality, make the welding surface smooth and well-distributed, and reduce following machining amount after being welded. Tensile test shows that ductile fracture happens in parent metal place, far away from the weld. The maximum tensile stress is 809 MPa, and the mechanical performance of the weld is good.