In order to investigate the influence factors of the fiber welding modes for 304 stainless steel, a group of experiments with the laser power, welding speed and defocus distance as the variables are designed in accordance with the idea of an uniform experimental design. By the analysis of the formed welds, the characteristic forms of the heat-conduction welding and the deep-penetration welding are obtained in the fiber laser welding process. The relationships among the weld mode, each welding parameter and their interactive action are calculated quantitatively, and the influence factors of the welding modes are determined. The results show that, there are four typical weld formation forms determined by the heat input density for the 304 stainless steel fiber laser welding. The weld has an arc shape when the heat input density is smaller than 43.04 J·mm -3 and the corresponding welding mode is the heat-conduction welding. When the heat input density is larger than 66.82 J·mm -3, the weld has a nail-head or a nail shape and the corresponding welding mode is the deep-penetration welding. Meanwhile, there exists an intermediate mode when the heat input density is between the two values for the above welding modes.