Ni/Ni3C nanoparticle-decorated ultrathin porous graphitic carbon nitride (gCN) nanosheets with a C-rich system show the highest catalytic activity compared with raw gCN nanosheets and gCN-Ni3C. As shown in UV-vis spectra, X-ray photoelectron spectroscopy results, and Mott-Schottky plots, the C-rich aromatic monomer 2,4,6-trihydroxy-1,3-diazine shows an excellent effect on changing the electronic structure of raw gCN nanosheets. Benefitting from Ni/Ni3C nanoparticle unique morphology and metallic characteristic, a larger active surface area and more rapid electron transfer were obtained. Moreover, gCN nanosheets could prevent Ni/Ni3C nanoparticle aggregation; thus, a better catalytic performance was obtained. By further optimizing the reaction parameters, the amount of hydrogen evolution could reach up to 9031.9 mu mol/g in 5 h. This design may broaden the application of metal carbides in the field of clean energy production and facilitate the alleviation of the current energy crisis.