Mg-Fe-Al composite spinel was synthesized via carbon thermal reduction method at 1550℃ with a size of ?20 mm×10 mm. The phase composition and the microstructure of the prepared composite spinel was characterized by X ray diffractometer and scanning electron microscopy respectively. The lattice constant and the unit cell volume of the Mg-Fe-Al composite spinel were calculated by combining the interplanar spacing of the specific crystal plane. Potassium dichromate volumetric method was used to determine the content of ferrous oxide in the composite spinel. Results show that two kinds of Mg-Fe-Al composite spinel with different morphology were produced,of which the lattice constant and unit cell volume are larger than those presented in standard cards for MgO·Al₂O₃ and the main peaks of which shift to the lower angle side. A small amount of MgO and Al₂O₃ were reduced by C forming Mg- and Al-vapor, which then react with CO and O₂ depositing as acicular MgO·Al₂O₃. The presence of liquid phase at high temperature promoted the formation of Al_15.99Mg_7.64Fe_0.37O₃₂ phase. A large amount of MgO and Al₂O₃ reacted directly to form granular MgO·Al₂O₃, Fe²⁺ and Fe³⁺ diffused into MgO·Al₂O₃ crystallites to form Mg_8.13Al_14.25Fe_1.13O₃₂ phase. The content of FeO in the composite spinel was 2.38% measured by potassium dichromate volumetric method.