Poly(lactic acid)-poly(ethylene glycol) copolymers were synthesized under the catalysis of multi-SO3H-functionalized ionic liquid. Compared to the ordinary ionic liquids and the traditional Lewis acid catalysts, the ionic liquids with multi-sulfonic acid groups were more catalytically active, and the reaction conversion rate reached up to 97.8 %. The molecular weight of the resulting copolymer was 5.69 x 10(4) g mol(-1) and the degree of crystallinity was 42.9 %. The copolymers were also of higher hydrophilicity and better mechanical properties. The reaction kinetics of copolymerization was analyzed. The intrinsically high catalytic activity of multi-SO3H groups originated from the lower activation energy and the higher free acidity. The recovering catalytic activity of the multi-SO3H ionic liquid catalyst was higher, suggesting that it is a recyclable, environmentally friendly catalyst.