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The electrochemical behavior of iron in H2S - containing H2SO4 solutions has been studied by polarization curve method. The stable adsorption orientation and optimal adsorption distance of H2S and HS- on the crystal plane of iron at cathodic and anodic potential were determined by CNDO/2 method. By calculating the total energy, binding energy and net charge distribution of the absorptives, the adsorption behavior of H2S and HS- on the crystal plane of iron and the bonding state between iron and H2S/HS- at cathodic and anodic potential have been investigated. The following were inferred from the above results. At anodic potential, the adsorption of large amounts of HS- on the crystal plane of iron in parallel direction through sulfur atom, which transferred the most of negative charge from HS-, promoted the dissolution of iron. While at cathodic potential, the adsorption of small amounts of H2S on the crystal plane of iron in parallel direction through hydrogen atom and the negative charge transfer from iron to hydrogen atom in H2S interacting with iron promoted the evolution of hydrogen. The experimental results are satisfactorily explained with the above theoretical calculation.