Unsaturated alcohols, usually produced from selective hydrogenation of unsaturated aldehydes, are important fine chemical intermediates used to synthesize pharmaceuticals and flavoring materials. Acrolein, the smallest member in alpha, beta-unsaturated aldehydes, is the model system for studying selective hydrogenation of alpha, beta-unsaturated aldehydes. So far most theoretical work is about adsorption and reactions of acrolein and its related species on surfaces. In the present paper we systematically studied the geometries, electronic structures, stability and transformation of various species derived from stepwise hydrogenation of acrolein in the gas phases. We identified the most stable intermediates for each system and determined the energy barrier for intermolecular conversion between isomers for various species with different content of hydrogen. All these results are valuable and informative for understanding the surface chemistry of hydrogenation of alpha, beta-unsaturated aldehydes.