The selective hydrogenation of unsaturated aldehyde in a gas-liquid-liquid-solid four-phase batch reactor was studied. Pd/C was used as the catalyst and an aqueous alkaline solution was used as the cocatalyst. The effect of the concentration of the aqueous alkaline on the selectivity for the saturated aldehyde is mainly discussed in this paper. A low concentration of the aqueous alkaline solution improved the selectivity for the saturated aldehyde. On the other hand, a high concentration of the aqueous alkaline solution acted as a catalyst for aldol condensation. This decreased the selectivity.