Developing bifunctional electrocatalyst with low-cost, high-activity, and durability towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is the key for large-scale hydrogen production. In this paper, a novel one-dimensional amorphous nickel molybdenum phosphate nanoarray (NiMoPOx) is prepared on nickel foam via two-step reations. NiMoPOx exhibits excellent electrocatalytic activity than the state of the art Pt/C and RuO2. For overall water splitting reaction, NiMoPO. electrode couple achieves 1.55 V at 10 mA cm(-2) for prolonged 50 h operation in 1.0 M KOH. The excellent bifunctional electrocatalytic properties come from the insitu electrochemical surface self-tuning of NiMoPOx "precatalyst". During the self-tuning process, the surface of amorphous NiMoPOx is in-situ transformed to metallic Ni and Ni(OH)(2) nanoparticles under HER and OER, leading to the formation of special heterostructure hybrid materials. Further investigations indicate that the insitu produced metallic Ni nanoparticles favor the generatation of adsorbed H atom in water dissociation, therefore enhance the hydrogen evolution kinetics and activity. Meanwhile the in-situ formed Ni(OH)(2) nanoparticles become the actual surficial species to promote oxygen evolution reaction. This self-tuning method provides a new idea to develop high-performance electrocatalysts for some sustainable energy conversion systems, such as water splitting and metal-air batteries.