Acetalation of formaldehyde (HCHO) with dialkyl formal or aliphatic alcohol to prepare polyoxymethylene dialkyl ethers (RO(CH₂O)_nR, n ≥ 1) catalyzed by Brønsted-acidic ionic liquids has been developed. The correlation between the structure and acidity activity of various ionic liquids was studied. Among the ionic liquids investigated, 1-(4-sulfonic acid)butyl-3-methylimidazolium hydrogen sulfate ([MIMBs]HSO₄) exhibited the best catalytic performance in the reaction of diethoxymethane (DEM₁) with trioxane. The influences of ionic liquid loading, molar ratio of DEM₁ to HCHO, reaction temperature, pressure, time, and reactant source on the catalytic reaction were explored using [MIMBs]HSO₄ as the catalyst. Under the optimal conditions of n([MIMBs]HSO₄):n(DEM₁):n(HCHO) = 1:80:80, 140 °C, and 4 h, the conversion of HCHO and selectivity for DEM_2–8 were 92.6% and 95.1%, respectively. The [MIMBs]HSO₄ catalyst could be easily separated and reused. A feasible mechanism for the catalytic performance of [MIMBs]HSO₄ was proposed.