Quartz crystal microbalance (QCM), a nanogram-level sensor, offers the advantages of high detection accuracy, wide humidity detection range, low cost, and simple fabrication method via the addition of humidity-sensitive materials in sensitive areas to detect humidity under gas phase conditions. To enhance the performance of hygroscopic materials and expand the applications of QCM-based humidity sensors, a QCM humidity sensor based on a chitosan (CS) substrate and polyvinylpyrrolidone (PVP) film-forming mechanism was constructed. The experimental results show that the response/recovery time of the sensor is 13 s/14 s, the sensitivity reaches 57.84 Hz/%RH in the detection range of 11%RH-97%RH, and the maximum frequency fluctuation is less than 10 Hz within 30 days in repeated experiments.