The synergistic effects of bifunctional catalysts in both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) must be better understood. Herein, the 3D hierarchical heterostructure consisting of NiCu surface layer assembled on the FeNi2S4 (or Ni-Fe layered-double-hydroxide, NiFe-LDH) is constructed on the nickel foam (NF) and the bifunctional catalytic effects of HER/OER are investigated in an alkaline solution. Although the high OER activity of NiFe-LDH and HER activity of NiCu/NF, NiCu/FeNi2S4 heterostructures is demonstrated by the small overpotentials of HER/OER (306 mV at a current density of 10 mA cm-2), in comparison with NiCu/NF (315 mV) and NiCu/NiFe-LDH (357 mV). An ultra-low cell voltage of 1.54 V is accomplished in overall water splitting. The high-performance bifunctional catalytic effects can be attributed to the synergistic effects of reversible surface reconstruction in the presence of S element, enabling the formation of the NiO/Ni interface at the low potential for HER and high oxidation states (Ni2+δ) at the high potential for OER. The results provide insights into the design of bifunctional electrocatalysts with high activity. •NiCu/FeNiS heterostructure for bifunctional catalytic properties.•To analyze the synergistic effects for bifunctional catalytic HER/OER.•Relationship between bifunctional catalytic properties and reversible surface reconstruction.