ABSTRACT: This paper describes a method for fabricating humidity sensors by cross-linking poly(ionic liquid)s (PILs) in the cellulose fibers of printing paper via an in situ photoinitiated “thiol-ene” click reaction. The ionic liquid and monomers could permeate into cellulose fibers with solvent and then polymerize. The porous framework of printing paper could be remained before the PILs content exceeds 24 w.t.%. The resultant sensors inherit both the strong hydrophilic property and high ionic conductivity, which are the most important factors for a high performance impedance-type humidity sensor from paper cellulose and PILs, respectively. When relative humidity (RH) goes up, the cellulose fibers adsorbed abundance water molecules and, then, the ion could ionize from PILs and participate in conduction process, leading to a significant decrease of impedance modulus. The introduction of PILs improved the sensitivity, hysteresis, response speed and the stability of paper-based devices. The sensitivity of the optimal paper-based device increases to 961.3 from 408.2 after adding PILs, and the response time was shortened to 25 s, which is prominent among the reported paper-based humidity sensors.