In order to investigate the effect of 7075 aluminum alloy on the microstructure and mechanical properties of AZ91 magnesium alloy, the optical microscope, scanning electron microscope, X-ray diffraction method and universal material testing machine were used to analysis the microstructure and the mechanical properties of AZ91 magnesium alloy. The results indicate that the addition of 7075 aluminum alloy can refine the as-cast microstructure of AZ91 alloy obviously. The volume fraction of Mg17Al12 phase is decreased, and Al6Mn phase is formed when the addition content of 7075 aluminum alloy exceeds 4%. The tensile strength and elongation of AZ91 magnesium alloy increase with increasing of the addition amount of 7075 aluminum alloy. When the addition of 7075 aluminum alloy is 4%, the tensile strength and elongation of AZ91 alloy are at the maximum value, which are 186 MPa and 8.2%, respectively