Carbon　nitride　(CN)　polymers　exhibit　tunable　and　fascinating　physicochemical　properties　and　are　thus　an　essential　class　of　photocatalytic　materials　with　potential　applications.　Although　significant　progress　has　been　made　in　the　fabrication　of　CN,　the　preparation　of　metal-free　crystalline　CN　via　a　straightforward　method　remains　a　considerable　challenge.　Herein,　we　describe　a　new　attempt　to　synthesize　crystalline　carbon　nitride　(CCN)　with　a　well-developed　structure　through　regulation　of　the　polymerization　kinetics.　The　synthetic　process　involves　the　pre-polymerization　of　melamine　to　remove　most　of　the　ammonia　and　further　calcination　of　the　pre-heated　melamine　in　the　presence　of　copper　oxide　as　an　ammonia　absorbent.　Copper　oxide　can　decompose　the　ammonia　produced　by　the　polymerization　process,　thereby　promoting　the　reaction.　These　conditions　facilitate　the　polycondensation　process　while　avoiding　carbonization　of　the　polymeric　backbone　at　high　temperatures.　Owing　to　the　high　crystallinity,　nanosheet　structure,　and　efficient　charge-carrier　transmission　capacity,　the　as-prepared　CCN　catalyst　shows　much　higher　photocatalytic　activity　than　its　counterparts.　Our　study　provides　a　novel　strategy　for　the　rational　design　and　synthesis　of　high-performance　carbon　nitride　photocatalysts　by　simultaneously　optimizing　polymerization　kinetics　and　crystallographic　structures.　©　2023　Wiley-VCH　GmbH.