It is a hot topic to seek cheap and efficient cocatalyst to improve the activity of graphitic carbon nitride (g-C3N4) in photocatalytic water splitting to produce hydrogen and photoelectrochemical (PEC) performance. Herein, we prepared successfully the novel 0D/2D heterojunction by the modification of Ni2P quantum dots (QDs) as co-catalyst on the surface of ultrathin g-C3N4 layer, which can greatly enhance the photocatalytic hydrogen production and PEC performance under visible light owing to the improvement of separation efficiency of photo generated charge carriers and visible-light absorption capacity. Surprisingly, the optimum amount of Ni2P loaded on the g-C3N4 is 3 wt%, whose hydrogen production rate is 1503 mu mol h(-1) g(-1) being far superior to that of g-C3N4 decorated by 3 wt% Pt (560 mu mol h(-1) g(-1)). Moreover, the photocurrent response value of Ni2P/g-C3N4 photocatalyst is over 11 times and 3 times that of pure g-C3N4 and Pt/g-C3N4, respectively. What's better, the stable photocatalytic H-2 evolution and PEC performance of Ni2P/g-C3N4 demonstrates its high stability and reusability resulting from Ni-N coordination on the surface of g-C3N4. This work provides valid evidence for the development of cheap, efficient and durable cocatalyst actting on the g-C3N4, opening up new opportunities and possibilities for dual function application.