A porous 0D/2D C3N4 nanocomposite (PCN) has been successfully prepared by a simple and green one-pot photochemical oxidation method for the first time. Notably, the PCN samples are easily one-pot synthesized in hundreds of grams in the laboratory. The structure features, morphologies and photoabsorption performances of the samples are characterized in detail. The photocatalytic activities are evaluated by hydrogen evolution under visible light irradiation. It was proven that the green chemical scissor, produced by the photodecomposition of H2O2, can tailor the texture of layered C3N4 to form porous C3N4 nanosheets and C3N4 quantum dots (QDs) without doping heteroatom merely through the destruction of part of triazine rings, and the as-prepared PCN sample still keeps the basic chemical structure of g-C3N4. The band alignment between CN QDs and C3N4 nanosheets as well as the porous structure will facilitate the separation of charge carriers and provide more active sites. Therefore, the photocatalytic hydrogen evolution rate of PCN is over 30 times higher than that of the parent bulk g-C3N4. The activity is also higher than that of ultrathin pore-free C3N4 nanosheets and porous C3N4 nanosheets. Moreover, the photocatalytic activity of PCN could be further improved by modifying with carbon quantum dots (C QDs). Finally, a possible photocatalytic reaction process is proposed.