A novel transparent and anhydrous proton conductor, which can be used in solid electrochromic device (ECD), was prepared by mixing poly (vinyl alcohol) (PVA) with ammonium dihydrogen phosphate (NH4H2PO4). X-ray diffraction, differential scanning calorimeter (DSC), and Infrared spectra were used to characterize the structure of PVA/xNH4H2PO4 composite membrane. Proton conductivity of the composite membranes was studied by the complex impedance method. The proton conductivity of the composite membranes increases with increasing temperature and increases with increasing phosphate doping-level at first and then decreases with increasing phosphate content after a certain value of x. The highest proton conductivity is near the area of x=0.067. The transmittance of the complex membranes always decreases with increasing doping level of phosphate.