Chemical properties of the supported ruthenium catalysts as well as their performances for hydrogenation of carbon dioxide(CO2)to formic acid have been investigated systematically. The surface properties of γ-Al2O3 supported ruthenium catalysts were explored by various characterization techniques（FT-IR and XPS). Supports with abundant hydroxyl groups and the well interaction between the active ruthenium components with hydroxyl groups on supports surface are required to obtain the hydroxide ruthenium catalyst. Catalytic performances of the γ-Al2O3 supported ruthenium catalysts were found to be strongly dependent on active ruthenium species dispersion and the chemical state. Favorable dispersion of ruthenium species on γ-Al2O3 was found to be advantageous to the hydrogenation of supercritical CO2 to formic acid，while the appearance of crystal RuO2 species which results from the too high ruthenium loading and the too strong basicity of catalyst preparation solution restricts to the yield of formic acid. An optimization of supported ruthenium catalyst for the hydrogenation of supercritical CO2 to formic acid was observed over γ-Al2O3 supported 2.0 wt％ ruthenium with catalyst preparation solution PH value12.8 and NH3·H2O as titration solvent catalyst in this work.