Polyoxymethylene dimethyl ethers (PODEn) are widely applied as diesel additives in engines. Ionic liquids (ILs) replace traditional liquid acids as catalysts in chemical processes. A series of pyrrolidinonium-based BrOnsted acidity ILs were synthesized, investigated, and employed as catalysts for the synthesis of PODEn from methylal and trioxane for the first time. The Hammett function values were measured to uncover the connection between catalytic performance and acidity-activity of the ILs considered. The optimal experimental conditions for the synthesis of PODEn were determined. The maximum values of both the conversion of raw material and the selectivity of PODE3-8 were obtained with 1-octyl-2-pyrrolidinonium trifluoromethanesulfonate ([NOP][TFO]) as the catalyst. [NOP][TFO] provides greater selectivity of PODE3-4 than the traditional catalysts such as H2SO4 and CF3SO3H.