VO2 films on muscovite (011) substrate were prepared by the inorganic sol–gel method. Their thermal stability was investigated in detail by annealing them in air at different temperatures and for different time periods. The crystalline structure, morphology, and stoichiometry of the films before and after air annealing were investigated by X-ray diffraction, scanning electron microscope, and X-ray photoelectron spectroscopy; Fourier transform infrared spectroscopy was used to study the changes in their thermochromic properties. The results indicated that the VO2 thin film was quite stable in air below 200 °C, which showed steady phase transition properties. However, when the air annealing temperature increased (above 200 °C), the phase transition temperature (T t) of the VO2 films increased and VO2 was oxidized to V2O5 gradually, resulting in a decrease in the phase switching efficiency of VO2 films. Moreover, the relationships between air annealing temperature and phase transition temperature, and air annealing time and phase transition temperature were investigated.