In one embodiment of the invention a method to prepare sucralose-6 -acylate through chlorinating sucrose-6-acylate by BTC in the process of sucralose preparation is disclosed. In this embodiment a Vilsmeier reagent is firstly prepared below 0° C. by dissolving BTC in DMF or in component solvent, containing DMF, toluene, dichloroethane, chloroform and carbon tetrachloride. Consequently, sucrose-6-ester was chlorinated by Vilsmeier reagent. BTC can also be dissolved in one or several organic solvent such as toluene, dichloroethane, chloroform and carbon tetrachloride, and added to a DMF solution of sucrose-6-acylate for chlorination. Sucralose was prepared through de-esterifying the obtained sucralose 6-ester using sodium methoxide/methanol or sodium ethoxide/ethanol.