Molybdenum polyoxometalate (PMo)/silica mesoporous composite thin films, which can be applied as multifunctional materials for photochromic and electrochemical applications, were prepared by impregnating PMo into amino-functionalized mesoporous silica thin films. The composite thin films possess excellent reversible photochromic properties and change from colorless to blue under ultraviolet (UV) irradiation. It is shown in the study that intervalence charge transfer and ligand-to-metal charge transfer are the main reasons for photochromism. After UV irradiation, the charge transfer occurs by the reduction of heteropolyanions accompanying the formation of heteropolyblues with multivalence Mo (+6, +5), and the bleaching process of composite thin films is closely related to the presence of oxygen. Moreover, the composite thin films deposited on the indium tin oxide (ITO) substrate can be used as the electrode and have many advantages, including simple fabrication, fast response, and good stability. The modified ITO electrode retains the electrochemical properties of PMo, can catalyze the electroreduction of the BrO3−, and may be used as the current sensor for the BrO3−.