Based on the density functional theory, influences of Al doping on stability and electronic structure of MgxTi1-x alloys and their hydrides were investigated. By calculating the formation energies of Mg-Ti-Al system, it is found that the best effect was obtained when the Ti-Al ratio was fixed at 1: 1, where the metal alloy was most stable, and was helpful to reversibly hydrogenate. Moreover, the partial substitution of Al for Ti atoms decreased the stability of the hydrides and improved the hydrogen storage properties. The analyses of the density of states, electron density and bond length showed that the improved properties of MgxTi1-x alloys and their hydrides with Al doping are due to the decrease in the number of bonding electrons and the weakening of Mg-H and Ti-H interactions in doped systems.