The efficient and sustainable production of high-purity hydrogen gas through electrochemical water splitting calls for robust and bifunctional catalysts to accelerate the two half reactions of water splitting. Herein, we in-situ craft ultrathin CoNi-sulfides nanosheet (similar to 2.2 nm in thickness) arrays-anchored MnO2 nanoparticles (similar to 3.2 nm in diameter) (denoted U-CoNi-S-NSA/MnO2) via a spontaneous redox process between CoNi-sulfides nanosheet and MnO4- anions at ambient temperature. The hierarchical U-CoNi-S-NSA/MnO2 nanocomposites are then directly employed as self-supported catalysts for the two half reactions of water splitting, showing excellent activity with small overpotentials of 170 mV for oxygen evolution reaction and 67 mV for hydrogen evolution reaction to achieve 10 mA cm(-2), respectively. Moreover, an efficient water electrolyzer through using U-CoNi-S-NSA/MnO2 as both anodic and cathodic catalysts is fabricated, which achieves current density of 10 mA cm(-2) at a small voltage of 1.51 V over a long-time operation of 20 h. This outstanding performance is markedly superior than that of precious Pt/C//RuO2 counterpart (1.61 V). Therefore, the as-synthesized hierarchical nano composites are promising candidates for cheap and efficient water splitting.