Seeking highly efficient non-preference electrocatalytic materials that serve photoelectrochemical (PEC) water splitting in acidic systems is expectant in the context of environmentally friendly production. We designed Ni2P electrocatalysts synthesized in oil phases via the hot-bubbling method with superb stability in air and sulfuric acid solution for PEC, which were found with excellent hydrogen evolution performance. A tunable particle size and highly exposed (001) planes of Ni2P nanocrystals were achieved. The designed catalysts achieved a notable promotion in the hydrogen evolution reaction activity compared to that of Ni2P synthesized in the water phase. More specifically, the electrode prepared by self-assembled Ni2P nanoparticles was found to have decent over-potential of η10 = 164 mV in darkness and was further decreased to 129 mV with irradiation of visible light. The cyclic stability tests manifested brilliant durability in 0.5 M H2SO4. Measurement of the transient photocurrent response and PEC water splitting catalytic performance indicated that the Ni2P had high carrier concentration upon irradiation, lower carrier recombination probability, and prolonged photo-response lifetime (3.03-3.14 s).