Two　kinds　of　mullite　solid　solution　with　different　compositions,　e.g.:　Al4.59Si1.41O9.70　and　Al4.75Si1.25O9.63,　were　synthesized　using　waste　aluminum　sludge　as　raw　materials　by　changing　the　batching　ratio　of　raw　materials.　The　mechanism　of　the　formation　of　mullite　solid　solutions　was　discussed　from　the　viewpoint　of　crystal　defects,　and　the　detailed　defects　formula　and　reaction　equations　were　given.　The　lattice　parameters　were　determined　and　the　amount　of　crystalline　phases　was　calculated　by　Philips　X＇pert　plus　and　Rietveld　Quantification　software,　respectively.　The　microstructures　of　crystalline　phases　were　observed　with　scanning　electron　microscopy.　The　results　show　that　the　highest　content　of　mullite　solid　solution　is　92%,　and　the　lattice　parameter　a　increases　with　the　increase　of　Al2O3　content,　but　the　changes　of　parameters　b　and　c　are　irregular.