Large surface area, like quantum confinement effect also caused by particular nano level size of quantum dots (QDs), brings fantastic possibility for humidity sensing. High concentration of surface adsorption site initiate response increasing. Porosity between QDs stacking up fast water vapor penetration and flowing away. Here, a quartz-crystal microbalance (QCM) humidity sensor was prepared using the indium oxide (In2O3) QDs, synthesized via solvothermal method. After In2O3 QDs directly spin-coating onto the QCM, an annealing process taken place to remove organic long chains and expose more moisture adsorption sites on the surface of the QDs. The annealed QCM humidity sensor exhibited high sensitivity (56.3 Hz/%RH at 86.3% RH), with a fast response/recovery time (14 s/16 s). Long carbon chains break down and hydrogen-bonded hydroxyl groups chemisorpted to the QDs. Chemical reaction was reduced by these chemisorpted hydrogen-bonded hydroxyl groups. Mass changing was mostly caused by fast multilayer physiorption. So the transducer can effectively and precisely monitor the moisture of person’s breathing. In2O3 QDs modified QCM sensors demonstrating its promising humidity sensing applications in daily life.