Abstract: Structures, properties and chemical bonds and of ferro aluminates of cement minerals are studied by SCF-X_α-DV method, one of the molecular orbital calculating method in quantum chemistry. The calculated results show that the net charge of Al is higher than that of Fe, the covalent bond of Al-O is weaker than that of Fe-O, and with the increase of Al element content, the net charge of Al becomes higher, the covalent bond of Al O shows weaker, and the energy level of the lowest unoccupied molecular becomes lower, those are just the reason that with the increase of Al element, there is superior hydration activity of ferro aluminates.