A new solid high-proton conductor, substituted heteropoly acid with Dawson structure H₇[In(H₂O)P₂W₁₇O₆₁]·23H₂O, has been synthesized by the degradation/ion exchange/freezing method. The pH of the synthesis reaction was given. The product was characterized by chemical analysis, IR, UV, XRD and TG-DTA. The determination of conductivity shows that H₇[In(H₂O)P₂W₁₇O₆₁]·23H₂O is an excellent solid high-proton conductor with conductivity of 1.34 × 10⁻³ S cm⁻¹ at 18 °C, and 70% relative humidity (RH). Its activation energy is 37.72 kJ mol⁻¹, which suggests that its mechanism of proton conduction is the Vehicle mechanism.