Recently, Co2P has been deemed to be a low-cost and fruitful activator for promoting photocatalytic hydrogen (H-2) production activity for graphitic carbon nitride (g-C3N4). Nevertheless, almost all reported synthesis methods undergo complex processes and the emission of highly toxic phosphorus gas (PH3). Herein, we present a plain and green method to prepare Co2P/g-C3N4 composite through in-situ synthesizing Co2P nanocrystal on g-C3N4 nanosheet under coheating treatment. The Co2P/g-C3N4 composite is demonstrated to show chemical bonding between Co2P and g-C3N4 nanosheet. Interestingly, Co2P/g-C3N4 presents 6.3 times enhanced photocatalytic H-2 yield of 4752 mu mol/ gill than that for an individual g-C3N4 nanosheet, resulting from an improvement in generating and separating electron and hole pairs by the chemical bonding between Co2P and g-C3N4 nanosheet. Notably, the performance exceeds a majority of nonmetal cocatalyst decorated g-C3N4 nanosheets. More importantly, this composite photocatalyst shows an excellent cycle stability with only 0.4% decrease in hydrogen evolution rate after four cycles. This article delivers a plain means to synthesize Co2P/g-C3N4 composite photocatalyst, easily expanding to other transition metal phosphides.