Polymeric carbon nitrides (PCNs) are candidates for metal-containing photocatalysts due to the unique conjugated structures. However, the low activity of PCNs due to their complex microstructure and lack of active surface functional groups can be boosted by finely controlling the active heptazine structure and chemical modification. We herein propose a self-assembly strategy to synthesize PCNs with adjustable microstructure and composition, using benzoic acid and melamine as raw materials. Addition of benzoic acid not only controlled morphology, heptazine structure and band structure, but also introduced hydroxyl and carboxyl groups. The introduced polar groups boosted the formation of singlet oxygen and superoxide anion, improving the photocatalytic activity in selective oxidation of benzylamine; the regulation of heptazine units enhanced the separation and mobility of charge carriers, promoting the activity of hydrogen evolution. Our work provides a strategy for the design of metal-free photocatalysts for the selective synthesis of organics and hydrogen production via photocatalysis.