Ruthenium carbon (Ru/C) catalyst is obtained by co-electroreduction of carbonate and ruthenium ions in mixed sodium carbonate, sodium chloride, calcium oxide molten salts (Na₂CO₃-NaCl-CaO) from carbon dioxide (CO₂) and hydrated ruthenium chloride (RuCl₃*nH₂O). Irregular particles with Ru loaded on carbon, Ru particles coated by carbon and nanocarbon fibers filled with Ru are prepared. Ru/C catalysts with different Ru content of 16.0 wt%, 36.5 wt% and 76.8 wt% are obtained. With the increase of Ru content, the overpotentials (-10 mA cm⁻²) for HER are respectively 110.8 mV, 198.4 mV, 31.9 mV in strong acid electrolyte (0.5 M H₂SO₄) and 50.2 mV, 197.9 mV, 16.8 mV in strong alkaline electrolyte (1.0 M KOH). Ru/C prepared at 800 °C, 3.0 V and RuCl₃*nH₂O content of 1.0 wt% for electrolysis 6 h are of more advantageous for application. The stability of Ru/C catalyst still needs to be improved in 1.0 M KOH. In final, Ru/C electrocatalysts for HER can be successfully prepared by a green route of molten mixed salt from RuCl₃*nH₂O and CO₂.