Improving the selectivity of resistive gas sensors and understanding the mechanism of gas sensors are quite significant for the design and application of gas sensors in practical application. In this work, a high selective acetone sensor based on n–n homojunction by loading ZnO quantum dots (QDs) onto ZnO nanorod (ZnO QDs@ZnO nanorod) sensing material was fabricated. The microstructure and element analysis of ZnO QDs@ZnO nanorod were characterized by transmission electron microscope, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction. The experimental results showed that the sensor possesses excellent acetone selectivity (selectivity coefficient S₁/S₂ = 8.76, where S₁ represents the response value of the sensor to acetone and S₂ represents the response value of the sensor to interfering gas at the same concentration). The prepared sensor has the low detection limit of acetone (10 ppb). The sensing mechanism of enhanced selectivity for acetone was explored by in-situ Raman test and XPS, in which the n–n homojunction-associated conversion of lattice oxygen to surface adsorbed oxygen is proposed. Theoretical calculation demonstrates the transformation from lattice oxygen to adsorbed oxygen and the enhanced ability of acetone adsorption. The improved selectivity and the deep investigation of the mechanism provide an effective way for the design of highly selective gas sensing materials.