The invention provides a preparation method of aluminum sulfate. The preparation method includes: using a microspheric catalytic cracking dead catalyst of 1-200um in grain size, and enabling the dead catalyst to be in acid soluble reaction with sulfuric acid in certain conditions to obtain a solid-liquid mixture; centrifuging for solid-liquid separation to obtain an aluminum sulfate solution containing impurity ions; adding sodium dimethyl dithiocarbamate of a certain proportion into the solution to remove the impurity ions like Ni2+ and V5+ to obtain a clean aluminum sulfate solution; performing evaporative crystallization to obtain aluminum sulfate solid. Granular solid waste generated by petrochemical enterprises is adopted to prepare aluminum sulfate, so that the disposal problem of solid waste of the petrochemical enterprises is solved, aluminum resources are recycled, and the preparation method is economical and environment-friendly.