In this study, imidazole-based poly(ionic liquid)s (PILs) are introduced into acid-treated mesoporous silica by solid grinding and subsequent thermal polymerization methods. The purpose of treating mesoporous silica with hydrochloric acid is to increase the number of hydroxyl groups on the surface to fix more PILs. Because of the synergism of enhanced hydrophilicity and the mesoporous structure, the synthesized composite materials exhibited excellent capacities to adsorb and transport water vapor molecules and are suitable for low-humidity sensing applications. In the 7%–33% relative humidity (RH) range, the response of the as-prepared sensor reached 806% with a short response/recovery time of 8/13 s.