A quartz crystal microbalance (QCM) humidity sensor based on graphene quantum dots-chitosan (GQDs-CS) sensitive film has been successfully fabricated in this work. The composite ratio of sensitive material and the coating quality of the sensitive film were optimized. The morphological and chemical properties of GQDs-CS composites were inspected via scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier Transform infrared spectroscopy (FT-IR). The results manifest that prepared QCM sensor exhibits high response sensitivity (a frequency shift of −3291 Hz at 95% RH), short response/recovery time (36 s/3 s), tiny humidity hysteresis (˜1.6% RH), and excellent reproducibility, reversibility and long-term stability. Additionally, the electroacoustic parameters of the QCM sensors were also investigated by means of the impedance analysis method. Based on dynamic and static analysis of the sensor, it is speculated that the introduction of GQDs into CS enhanced the mechanical stiffness of the CS film, which could effectively improve the performance of QCM sensor.