A method for the synthesis of monodisperse polyethylene glycols by single chain extension strategy using cheap polyethylene glycols with degree of polymerization of 2 to 4 as raw materials based on macrocyclic sulfates is developed. With benzyl as the protecting group, the hydroxyl end of benzylated polyethylene glycols was extended by repeated nucleophilic ring opening of macrocyclic sulfate and subsequent hydrolysis reaction，and the benzylated monodisperse polyethylene glycol derivatives with degree of polymerization of 6 to 16 were obtained in 55%~86% yield. Finally, monodisperse polyethylene glycols with degree of polymerization of 6 to 16 were obtained in 95%~99% yield through debenzylation by catalytic hydrogenation. The structure of the intermediates and the final products were confirmed by NMR and MS. This method has the following advantages: 1) The reaction process is easy to monitor. 2) The reaction efficiency is high, and no polyethylene glycol impurities with degree of polymerization close to that of the target products were generated, so the intermediates and the final products are easy to be separated and purified.