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Carbon nitride quantum dots (CNQDs) were doped into melamine (MA) and silver (Ag) matrix yielding CNQDs@MA−Ag nanocomposites by the supramolecular self‐assembly route. To our surprise, the resulting mesoporous nanocomposites could present different morphologies (i. e., nanowires, nanoflowers, and nanosheets) and especially tunable visible‐light photocatalysis performances depending on the percentages of CNQDs used. It was discovered that the CNQDs@MA−Ag ones with the pinecone‐like structure (containing 30 % CNQDs) could exhibit the strongest visible‐light photocatalysis for splitting water to yield H2O2 with a production efficiency of 39.82 μmol ⋅ g−1 h−1, which is more than ten‐fold higher than that of CNQDs alone. Herein, the nano‐scaled Ag could facilitate the increased light harvesting and the separation as well as transmission of electron‐hole pairs of CNQDs resulting in the enhanced visible‐light photocatalysis of nanocomposites. More importantly, the pinecone‐like ones might additionally enable more exposed active sites of photocatalysis and larger mesoporous specific area of contacting dissolved oxygen in water, leading to the higher photocatalytic efficiency of H2O2 generation. This CNQDs‐based fabrication strategy may open a new door towards the design of a variety of photocatalysts with different morphologies and tunable photocatalysis performances for the efficient splitting water toward H2O2 evolution under sunlight. [ABSTRACT FROM AUTHOR]