Abstract: The study on non-penetrating laser welding of 3 mm-thick stainless steel sheets by fiber laser was conducted. The threshold interval of power density of stainless steel in fiber laser welding was obtained. The laser power of P and welding speed of v were changed in different modes, the effects of which on the color of weld backs and the weld sizes were contrasted and analyzed after recording the back colors of weld seam and measuring the weld cross-section sizes, which include surface seam width , middle weld width, and penetration. The results indicate that when the welding speed is 5 cm/s, the laser power density threshold interval for stainless steel fiber laser penetration welding is 3.19~3.61 kW/mm2. Either by increasing the laser power or by reducing the welding speed, to increase P/v at the same proportion, will enlarge the surface seam width and the penetration, but the former is more effective. As for the effects on middle weld width, the two modes are slightly different. And the changes of weld back colors are similar. When P/v remains constant, which means the power and speed are increased at the same proportion, the surface seam width is changed little and the middle weld width is increased. With regard to the penetration, it is increased first and then stays unchanged. In this mode, the colors of weld back are different. The mode of reducing welding speed is more effective in improving tensile-shear strength of overlap joint.