In this work, we performed density functional theory calculations to explore reaction pathways of ethanol to 1,3-butadiene on the surfaces of MgO(100) and MgO(110). The overall activation barrier and reaction energy were calculated for the Meerwein-Ponndorf-Verley (MPV) reaction and relative side reactions, cross aldol condensation, and hydrogenation reaction. It is verified that the acidity and basicity sites on MgO(110) are stronger than those on MgO(100), which is favorable for the MPV reaction and cross aldol condensation. On MgO(110), the MPV reaction takes place via the ethoxy and crotonaldehyde route due to dissociated ethanol and directly deoxidizes to produce 1,3-butadiene rather than the proton return reaction finally. The rate-determining step of the MPV reaction is proton transfer reaction on both surfaces. In addition, the cross aldol condensation is less likely to occur on MgO(100) and the hydrogenation side reaction is less likely to occur on MgO(110).