In the pursuit of energy efficient materials, vanadium dioxide (VO2) based smart coatings have gained much attention in recent years. In this paper, we investigate Al-doped VO2 films as thermochromic coatings on glass substrates by DC magnetron sputtering. It is found that adding Al3+ ions into VO2 films can generate groups of polygonal grains and nanowire clusters, apart from routinely reducing the valence and decreasing the grain size, through XRD, XPS, Raman shift and SEM results. For optical properties of Al doped films, a blue-shift of absorption edge in transmittance spectra has been introduced, which can significantly improve the luminous transmittance. Further, largely reducing phase transition temperature and enhancing solar modulation ability are also achieved. After carefully analyzing the relationship between the film characterizations and optical performance, we attribute the advanced theromochromic properties to the generation of nanowire clusters by the depositing conditions and the transformation effect of Al dopants. Such good thermochromic performances obtained by Al doping obviously overcome the drawbacks of undoped VO2 films for practical application. This work provides a considerable and new method of optimizing thermochromic properties of VO2 films as smart window coatings.