SnCl?·2H?O was introduced into polyacrylonitrile (PAN) nanofibers through a simple electrospinning method, and composite Sn nanoparticle–carbonized (Sn@C) nanofibers were prepared by two-stage calcination. After dispersing different amounts of composite nanofibers in a 2.5?wt% chitosan (CS) solution via ultrasonication, a series of CS/Sn@C composite humidity-sensitive layers were spin-coated on quartz crystal microbalance (QCM) sensor surfaces. The morphology, structure, and composition of Sn@C nanofibers were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption–desorption isotherm analysis combined with the Brunauer–Emmett–Teller (BET) method. The diameter of Sn@C nanofibers decreased from 180?nm to 95?nm upon decreasing the PAN content from 90% to 60%. The presence of metallic Sn nanoparticles, formed on the surface of Sn@C nanofibers, resulted in an increase in specific surface area from 5.1?m2·g?1?to 106.5?m2·g?1?. Using an electrospinning solution with PAN:SnCl?·2H?O (8:2 w/w) and a Sn@C dispersion (0.05?mg/mL) in 2.5?wt% CS solution, a QCM sensor with a fitting correlation coefficient of 0.9998 was fabricated. The sensor parameters were optimized through QCM testing under varying conditions. The sensor demonstrated fast response/recovery times: <?1.2?s (response)/50?s (recovery) in 11–33% RH and 67?s/37?s in 11–98% RH. Additionally, the CS/Sn@C-modified QCM sensor exhibited high repeatability, reversibility, and low humidity hysteresis.