High-valence cation doping was proved to be an effective method in reducing the high metal–insulator transition (MIT) temperature (θc). However, the mechanism of cation-dopants reducing θc is not quite clear and the reported data about the θc reduction induced by Nb dopants are dispersive. In this work, we prepared Nb-doped VO2 films by hydrothermal process and investigated the effects of Nb dopants on the MIT behavior. Valence variation of Nb dopants was observed in the Nb-doped VO2. The Nb5+ dopants elevate the valence band top to higher level than the Nb4+-containing sample. Correspondingly, the Nb5+ doping reduces θc in a higher rate of ∼21 ℃/at%Nb than the Nb4+-containing sample which has the decline rate of ∼18 ℃/at%Nb. The results imply Nb5+ dopants contributing excess electrons modify the band structure and consequently promoting the MIT transition. In addition, the Nb doping realizes the θc reduction almost without detriment to the optical properties of VO2, suggesting the promising applications in smart window and other optical switching devices.