Octadiphenylsulfonylsilsesquioxane (ODPSS) was synthesized through sulfonylation of octaphenylsilsesquioxane (OPS) with benzenesulfonyl chloride and the AlCl₃ catalyst in dichloromethane (CH₂Cl₂). By changing the solvent, catalyst, reaction time and molar ratio of reactants, the optimal reaction conditions were determined. It was found that OPS could be completely converted to ODPSS with a high yield. The product ODPSS was identified by FTIR, MALDI-TOF MS, ¹H NMR, ¹³C NMR, ²⁹Si NMR and elemental analysis. The reaction mechanism was analyzed and a sulfonylation model of OPS was established.