Mg-Al　spinel　was　prepared　by　using　waste　slag　from　an　aluminum　factory　and　(MgCO3)4·Mg　(OH)2·5H2O　as　the　main　starting　materials.　The　formation　mechanism　and　structure　of　spinel　were　investigated　by　X-ray　diffraction　and　analysis　software.　The　influence　of　composition　on　the　properties　of　spinel　was　also　discussed.　The　results　show　that　the　synthesized　Mg-Al　spinel　has　a　composition　of　(Mg0.68Al0.32)　(Al0.84Mg0.16)2O4,　which　exists　in　a　solid　state　solution　form,　and　the　variation　of　unit　cell　parameters　is　small.　When　the　molar　ratio　of　MgO　to　Al2O3　is　1.37,　the　mass　fraction　of　spinel　is　the　greatest,　93%,　and　the　spinel　sample　has　a　water　absorption　of　0.31%,　a　porosity　of　0.95%,　a　bulk　density　of　3.61　g/cm3　and　a　bending　strength　of　89.04　MPa.