Efficient overall water splitting over semiconductor photocatalyst is vital but difficult for resolving the energy and environmental crises. In this report, a novel transition metal phosphide Ni2P-based ternary CdS/Ni2P/g-C3N4 composite was constructed for achieving efficient overall water splitting activity under visible light (lambda > 420 nm). The H-2 and O-2 evolution ratios are 15.56 and 7.75 mu mol.g(-1) h(-1) over CdS/Ni2P/g-C3N4 with 3 wt % Ni2P, respectively, which is 4.02 times higher than that of binary type II CdS/g-C3N4. The separation efficiency of photocarriers of CdS/Ni2P/g-C3N4 is enhanced through greatly speeding up the transfer efficiency of electrons from the conduction band of g-C3N4 to that of CdS using Ni2P as electron-bridge, proved by the photoluminescence, transient photocurrent measurements and electrochemical impendence spectroscopy. The significant finding of this paper sheds light on the important role of transition metal phosphide as electron-bridge to connect the two conduction bands of type II semiconductor heterojuction for strengthening the separation efficiency of photocarriers.