Mesoporous Al2O3 with a high proton conductivity was synthesized. The conductivity was improved by doping inorganic salts, and increased significantly with doping chloride. X-ray photoelectron spectrometry and temperature-programmed desorption of ammonia methods were performed to investigate the effects of chloride on the surface properties. The results indicated that the surface chlorine has an important role in the change of the proton conductivity. The surface acidity, which was expressed by the concentration of the weak acid sites, was increased with doping chloride. The results showed that the proton conductivity of mesoporous Al2O3 has a close relation to the surface acidity.