Hydrogen,with the merits of zero emissions and high energy density,is one of the promising green energy sources.Seeking for high efficiency and low-cost catalysts is one of the key issues for hydrogen evolution and its practical applications.Nano-structured metal co-catalysts are widely used to improve the photocatalytic performance via surface electronic structure/properties optimization of the catalyst.Herein,we report ultra-fine(～ 1 nm) Cu clusters decorated hydrangea-like TiO2 systems for photocatalytic hydrogen evolution.The pristine hydrangea-like TiO2 support shows a promising perfor-mance of hydrogen evolution (1.8 mmol·h-1·g-1),which is ～ 10.7 times higher than that of the commercial P25(168 μmol·h-1·g-1).After ultra-fine Cu clusters decoration,a maximal hydrogen evolution performance(3.7 mmol·h-1·g-1) is achieved in the optimized system 6Cu-TiO2 (6 wt％).Experimental and theoretical studies demonstrate that the ultra-fine Cu clusters decoration could promote the charge separation and transfer process effec-tively.The Cu clusters also act as reaction sites for reduction of H2O to H2.These results are of great impor-tance for the study of Cu-based co-catalyst systems and also shed light on the development of other non-noble metal co-catalysts in photocatalysis hydrogen evolution.