Production of hydrogen using organic wastes via dark fermentation is a sustainable and cleaner technology. Cofermentation of multiple wastes with complementary characteristics is a promising approach to achieve high hydrogen productivity. In this study, sewage sludge and grass residue (mixed ratio of 3:7 based on volatile solids (VS)) were co-fermented for hydrogen production at various substrate concentrations (5-80 g/L based on VS). Results showed that hydrogen fermentation efficiency was improved when substrate concentration increased from 5 to 10 g/L, while exhibited a decreasing trend when substrate concentration further increased from 10 to 80 g/L. The substrate concentration was optimized to be 10 g/L for this co-fermentation system, and the maximum hydrogen yield and VS removal was 45.6 mL/g-VSadded and 13.7%, respectively. Investigation on process parameters revealed that high substrate concentrations caused the overloaded accumulation of metabolic products (lactate, acetate, propionate, butyrate, ethanol and ammonium nitrogen) and the significant decrease of pH, which inhibited the metabolic activity of microbes, and thus leading to the decrease of hydrogen fermentation efficiency.