A physical mixing is usually needed in order to load lithium chloride (LiCl) in a matrix. A method of in situ loading LiCl in the cross-linked polymer framework by click reaction is demonstrated in this work. Pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) and divinylbenzene (DVB) were taken as the monomers to construct stable cross-linked polymer matrix, while different contents of LiCl were introduced in the polymerization process for preparing humidity sensitive composites. The cross-linked composites with uniform distribution of LiCl were obtained. Humidity sensors based on the composites were fabricated and their sensing properties were systematically investigated. The optimum mass ratio of LiCl was determined to be 8 wt%, the corresponding sensor owns a variation of impedance modulus beyond two orders of magnitudes from 11% to 95% relative humidity (RH), a maximum humidity hysteresis of ～1.5% RH, and a short response and recovery time of 3.5 and 63 s, respectively. The humidity sensing mechanism of the obtained sensor was explored by the complex impedance spectroscopy and the equivalent circuit.