The atomic cluster models of Mg/CNT（carbon nanotube or nickel carbon nanotube） interface have been built in magnesium composite reinforced by （nickel） carbon nanotube. The interface electronic structures have been calculated by recursion method. The calculated results show that nickel enhances the stability of nanotube/matrix interface, improves the interface bond strength, since nickel makes the structure energy and atomic binding energy of nanotube/matrix interface lowered. The positive interaction energy of magnesium atoms in nickel film of interface causes the magnesium atoms in nickel film to repel each other and can not form magnesium atomic cluster, so they prefer to form ordering phase which consolidate the interface. The carbon and magnesium atoms have higher energy of position, which causes interface instability and flimsiness at carbon nanotube without nickel plating and magnesium interface. But when the carbon nanotubes have been plated by nickel, the energy of position of carbon and magnesium atoms are made lower and the interface gets more stable and tough.