Low humidity detection has been a great challenge in the field of humidity sensing. In recent years, polyelectrolytes have been proven to be promising materials for highly sensitive humidity sensors. In this work, cross-linked poly(ionic liquid)s (PILs) were prepared for low humidity detection. The strong hydrophilicity of PILs contributes to humidity sensing while the cross-linked structure guarantees the stability of the sensing film. The optimized sensor shows high response (48.0) in 5–35% RH, with short response/recovery time (1 s/10 s) and a small humidity hysteresis (0.2% RH). The possible sensing mechanism of the sensors was discussed.