Acidic poly(ionic liquid)s (PILs) with swelling ability were synthesized by free radical copolymerization of N-vinylimidazolium ionic liquids, divinylbenzene (DVB) and sodium acrylate (NaAA), and further acidification by sulfuric acid. The swelling ability of acidic PILs was greatly affected by cross-linker content and chain length of 3-alkyl-substituents on imidazolium. Cross-linked network structures could be observed from the cryogenic scanning electron microscopy (cryo-SEM) images of the swollen acidic PILs in formic acid. Acidic PILs with network structures in swollen state exhibited excellent activities in the esterification of cyclohexene and formic acid, and the catalytic activities were in positive correlation with their swelling abilities. Acidic PIL with 3-octyl-substituent and 2.5 mol% DVB (PIL-C8-2.5DVB-HSO4) had the highest swelling ability in formic acid and exhibited comparable catalytic activities with homogeneous catalysts such as sulfuric acid and p-toluenesulfonic acid. Acidic PILs with high swelling abilities in formic acid exhibited excellent catalytic activities in the esterification of cyclohexene and formic acid, which was comparable with homogeneous catalysts. Display omitted