In order to enhance the absorptivity of stainless steel surface in infrared band, nanosecond fiber laser was used to modify the morphology of stainless steel plate surface. The influences of process parameters on the geometrical dimensions of micro-crater, surface roughness and blackness were investigated systematically. The mechanism of laser enhanced absorptivity of stainless steel surface was obtained, and the method to improve the absorptivity and production efficiency was proposed. The results show that when other process parameters are fixed, the reduction of scanning speed leads to the overlap increase, inducing the depth of the micro-groove and the surface roughness of the sample increase. The mechanism of the absorptivity enhancement of stainless steel surface is the combined effect of laser-induced periodic surface structures (LIPSS) with light trapping effect and oxidation blackening, but the effect of light trapping of the LIPSS is more significant, therefore the absorptivity is proportional to the surface roughness Ra. When the scanning spacing is very small, the complex LIPSS with strong light trapping effect and the oxidation blackening caused by heat accumulation are produced, the surface roughness is very large, 92% absorptivity of infrared wavelength can be achieved. When the scanning spacing is suitable, the edge of the ablated micro-groove is not ablated by the pulse laser of next micro-groove, the sample surface is enough rough, high absorption and productivity can be obtained simultaneously. This research proposes a method to enhance the absorption of the metal surface.