First principle calculations are employed scheme to investigate the electronic structures and mechanical properties of AlCu3 with divacancy defects using the pseudopotential plane wave method. The defect crystal is constructed by removing the nearest Al or Cu atoms to form the double point vacancies. Calculated lattice constants agree well with the experimental data. Moreover, structural stabilities of crystals containing divacancy are reduced. Results reveal that the crystal with double Cu vacancy defects is more stable than that of Al. Calculations of defect formation energies indicate that the divacancy of Cu is easier to exist in AlCu3. Divacancy defects play an important role to improve the mechanical properties of AlCu3 and corresponding ductility, stiffness and plasticity are increased. Furthermore, chemical bonds become weaker since the divacancy are introduced. Hybridizations between orbits of crystals are analyzed to account for the interactions in perfect and defect structures. Accordingly, implications of these findings on the mechanism of divacancy are discussed.