In contrast to the results of sulfur atom adsorption, the adsorption of hydrogen sulfide on the Fe(100) surface has been studied using first principles method, which is based on the density functional theory (DFT). The structures, electronic properties were calculated by the generalized gradient approximation (GGA) for the coverage of 0.25 monolayer (ML). The results show that the H2S adsorbed on B2 site is stable and the adsorption energy is -1.23 eV and the structure of H2S is little changed. While the density of states (DOS) for the adsorption of hydrogen sulfide in the most unstable state after the adsorption at B1 and most stable adsorption at the site of B2 are analyzed. We have compared, under same conditions, the electronic properties of the sulfur atoms of the adsorbed hydrogen sulfide and a single sulfur atom adsorbed on Fe(100) surface. The adsorption effect is very weak for sulfur atoms in adsorbed hydrogen sulfide. At the same time, the density of states for the adsorption of Fe(100) surface was studied comparatively, and we found that the sulfur atom adsorption on Fe(100) showed a series of peaks that have discrete distributions generated by ferrous sulfide. It shows that the adsorption is given by sulfur atoms instead of molecules of hydrogen sulfide.