The synthesis of methylene diphenyl dimethylcarbamate (4,4'-MDC) from methyl N-phenyl carbonate (MPC) and formaldehyde (HCHO) was conducted in the presence of sulfonic acid-functionalized ionic liquids (ILs) as dual solvent-catalyst. The influences of the kind of anion in the ionic liquids, reaction conditions and the recycle of the ionic liquid on 4,4'-MDC synthesis reaction were investigated. In addition, the acid strength of ILs was determined by the Hammett method with UV-visible spectroscopy, and the acid strength-catalytic activity relationship was correlated. The activity estimation results showed that [HSO3-bmim]CF3SO3 Was the optimal dual solvent-catalyst. Under the suitable reaction conditions of 70 degrees C, 40 min, molar ratio of n(MPC)/n(HCHO) = 10/1 and mass ratio of W-ILs/W-MPC = 4.5/1, the yield of 4,4'-MDC based on HCHO was 89.9 % and the selectivity of 4,4'-MDC with respect to MPC was 74.9%. Besides, [HSO3-bmim]CF3SO3 was reused four times after being purified and no significant loss in the catalytic activity was observed.